Top 50 Solidity Interview Questions and Answers

Solidity is an object-oriented programming language used to implement smart contracts on blockchain platforms like Ethereum, which generates transaction records in the system. To excel in your journey toward top companies as a Solidity developer, you need to master some important Solidity Interview Questions to crack their Solidity Online Assessment Round and Solidity Interview Round. We have prepared a list of the Top 50 Solidity Interview Questions along with their answers to ace in interviews.

In this article, we have covered all the essential questions that are likely to be asked in interviews, whether you are a beginner embarking on your journey to become a Solidity Engineer/developer or a seasoned professional seeking a job change and aiming to refresh important concepts in line with current trends. Then this Interview guide is for you.

This Solidity Interview Questions article is designed for all skill levels, from beginners and intermediates to advanced candidates, encompassing a wide range of interview questions.

Solidity Interview Questions for Freshers

Explore the list of interview questions with answers for Solidity. Here you will find all the beginner-level questions that you will face during the interview.

1. What is Solidity?

Solidity is an object-oriented programming language developed by Christian Reitwiessner, Alex Beregszaszi, and several former Ethereum core contributors for implementing smart contracts on various blockchain platforms like Ethereum. It is used to implement smart contracts that implement business logic and generates a chain of transaction records in the blockchain system. 

2. What are the important features of Solidity?

• Solidity is a statically-typed object-oriented language.
• It is highly influenced by Python, C++, and JavaScript which runs on EVM.
• It is designed for implementing smart contracts.
• It can be used for implementing contracts for functionalities like Voting, Crowdfunding, Multi-signature wallets, etc.
• Solidity is a primary language for blockchain platforms.

3. What extension does the source file have in Solidity?

The Solidity contract file/ library file has an extension .sol. For example, Geeksforgeeks.sol. The SOL script is a file that is programmed in Solidity programming language in the same way files are created in C++ or any other programming language. The SOL file contains the solidity source code and is used to create smart contracts for the transactions on the blockchain.

4. What type of language is Solidity?

Solidity is a statically-typed high-0level object-oriented programming language designed for developing smart contracts that run on Ethereum Virtual Machine (EVM). 

5. Why is Solidity used in Blockchain?

Solidity is designed to develop smart contracts for transactions on the Blockchain. 

• It is used to create smart contracts that implement business logic.
• Smart contracts can be used for various purposes like multi-signature wallets, Voting, Bind auctions, Crowdfunding, etc.
• Smart contracts can be used to automate transactions on the blockchain.

6. What is a smart contract?

Smart contracts are computer programs stored on the blockchain that are executed when the predetermined conditions are met. 

• These are used to automate the execution so that all the participants can be immediately certain of the outcome.
• Smart contracts can automatically trigger the next action when the conditions are met.
• They remove the need for intermediaries to handle the transactions.
• Smart contracts don’t require any paperwork to process as they are digital and automated. 

7. What is a gas limit in Solidity?

A gas limit in Solidity refers to the maximum amount of gas a user is willing to spend on a transaction. When creating a transaction on the Ethereum Network user must specify the gas limit to ensure that they don’t spend more gas than they intend.

• If a transaction requires more gas than the specified gas limit, the transaction will be reverted, and the used gas will not be refunded.
• If a transaction requires less than the gas limit, the remaining gas will be returned to the user.

8. What is a variable in Solidity?

A variable in Solidity is a storage location that can contain values. These values can be changed during runtime. Variables are broadly classified as state variables and local variables.

• State variables are the variables that are declared outside any function in a contract and are permanently stored on the Ethereum blockchain.
• Local variables are the variables that are declared within the function and their scope is limited to that function.

9. What is an event in Solidity?

An event is an inheritable member of the contract, which stores the arguments passed in the transaction logs when emitted.

• Generally, events are used to inform the calling application about the current state of the contract, with the help of the logging facility of EVM.
• Events notify the applications about the change made to the contracts and applications which can be used to execute the dependent logic.

10. What is a library in Solidity?

Libraries in solidity are similar to contracts that contain reusable codes. A library has functions that can be called by other contracts.

• Deploying a common code by creating a library reduces the gas cost.
• Functions of the library can be called directly when they do not modify the state variables i.e. only pure and view functions can be called from outside of the library.
• The library does not have state variables, it cannot inherit any element and cannot be inherited.

Solidity Interview Questions for Intermediate

Explore the list of intermediate interview questions with answers for Solidity. So if you have experience spanning one or two years, then this section is very good for you to upscale your knowledge.

11. What is the constant function in Solidity?

A constant function does not change the state of the contract. Therefore, the constant function can be called from anywhere without concerning the security issues it will impose as the function will not affect the state of the contract.

12. Is it possible to use loops in Solidity?

Yes, one can use loops in Solidity. Solidity supports all necessary loops to make programming easier. One should be careful while using the loops in Solidity.

• Infinite loop results in the loss of the gas and failure to continue executing the contract.
• For loop cannot be used in solidity for iterating over an array.
• If the loop is updating the state of some variables of the contract then extra care should be taken. It should be bounded otherwise the contract can get stuck if the loop iteration is hitting the block’s gas limit.

13. What are hexadecimal literals in Solidity?

Hexadecimal literals are prefixed with hex keyword. These are enclosed in double or single quotes. For example, hex”2AB3F”. Hexadecimal literals have the same convertibility restrictions as string literals. 

14. What are the different Ether units available in Solidity?

The different ether units available in Solidity are wei, kwei, gwei, mwei, microether, milliether, ether.

15. What time units are available in Solidity?

Solidity has time units where the lowest time unit is second and other time units are seconds, minutes, hours, days, and weeks.

16. What are storage variables in Solidity?

Storage variables are the variables that are stored on the blockchain. These variables are permanent and are used for storing important data concerning the contract. They are also known as dynamic variables and are stored in the storage and cannot be changed. They are persistent.

17. What are source files in Solidity?

A solidity source file can contain any number of smart contracts or library definitions in a single file. 

• The source file can contain any number of contract definitions, imports, and pragma.
• It is recommended that a developer maintains different source files for each contract or library definition to have better code maintainability. 
• The solidity source file has .sol extension.

18. What are state variables in Solidity?

State variables are stored in the storage. These values are permanently stored in the contract’s storage. 

•  They store data directly on the blockchain.
• These are declared inside a contract and outside the function.
• These are expensive as they cost gas.
• These can be assigned one of the access modifiers, public, private, or internal.

19. What are the main components of a Solidity Smart Contract?

 The main components of a Solidity smart contract are:

• Pragma statement: This statement specifies the required version of the solidity compiler for the smart contract.
• State variables: These are used to store the contract’s state data persistently on the blockchain.
• Constructor: This is a special function that is used for initializing state variables and called at once at contract deployment.
• Functions: Functions define the logic and behavior of the contract.
• Modifiers: These are the reusable code snippets that can be added to functions to modify their behavior.
• Events: Events are custom data structures that emit transaction logs for external listeners to monitor contract activity and state changes.
• Inheritance: Inheritance allows a contract to inherit properties from a base contract thus enabling code reuse and abstraction.

20. What is special in smart contracts compared to other programs?

• Immutable: Once the smart contracts are deployed on the blockchain, their code cannot be altered. This makes the smart contract invulnerable to unauthorized access.
• Decentralized execution: The execution of smart contracts does not depend on a single authority but instead on multiple nodes that are spread around the world.
• Self-executing: Smart contracts are designed to execute predefined actions automatically when certain conditions specified in the contract are met. This reduces the need for intermediaries thus reducing the potential for human error.
• Tokenization: Smart contracts can be used to create and manage digital assets through the use of tokens.

21. List the difference between unit8 and unit16.

• uint8 stores a number of up to 2^8-1. It has 8 bits.
• uint16 stores number up to 2^16-1. It has 16 bits.

22. What are private and public variables in Solidity?

• Private variables can be accessed only within the contract that declares them. These are not accessible by an external entity nor by the contracts derived from the contract in which they are declared.
• Public variables can be accessed by any contract, function, or external entity. When these variables are declared in Solidity, the compiler automatically generates a getter function that allows any external entity or contract to read its value.

23. List the differences between view and pure functions.

Parameters	View Functions	Pure Functions
Definition	These are read-only functions, which ensures that state variables cannot be modified after calling them.	These do not read or modify the state variables, which return the values only using the parameters passed to the function or local variables present in it.
Data Access	read	None
Transaction Type	Call	Call
Ideal For	Getter functions to view data on the blockchain.	Defined to the scope of the function and not view data on the blockchain.

24. List the difference between memory and storage in Solidity.

Parameters	Memory	Storage
Definition	Memory in Solidity is a temporary place to store data.	Storage holds data between function calls.
Memory Type	The Solidity Smart Contract can use any amount of memory during the execution but once the execution stops, the Memory is completely wiped off for the next execution.	Storage is persistent, each execution of the Smart contract has access to the data previously stored in the storage area.
Usage	It is better to use Memory for intermediate calculations.	It is better to use Storage to store the final result.
What Does it Store?	Function arguments are stored in Memory.	State variables and Local Variables of structs, and arrays are always stored in Storage by default.

25. List the differences between struct and array.

Parameters	Struct	Array
Definition	A struct is a collection of heterogeneous data.	An array is a collection of homogeneous data.
Elements referenced by	Elements in the struct are referred to by their unique name.	Elements in an array are referred to by subscripts.
Data Type	It is a user-defined data type.	It is a derived data type.

26. How much is 1 gwei of Ether?

1 Gwei equals 0.000000001 or 10e-9 ETH. 1 ETH equals one billion (10e9) Gwei.

27. How much is 1 wei of Ether?

1 wei is equivalent to 10^-18 ETH. 1 ETH is equal to 1,000,000,000,000,000,000 wei/

28. How much is 1 kwei of Ether?

1 kwei is equal to 0.00000000 ETH.

Advanced Solidity Interview Questions for Experienced

In this section we have compiled all the Solidity advance level interview questions with answers.

29. What is the difference between Solidity and other programming languages?

• Programming languages like Python, C, and C++ are designed to create centralized applications. On the other hand, Solidity is designed to work with Ethereum Virtual Machine and is used to create smart contracts that work in a decentralized environment. 
• Solidity differs from the EVM-targeting languages like Serpent and Mutan in the way that Solidity supports complex member variables for the contracts, and supports inheritance.
• Solidity is a statically-typed language and Javascript is a dynamically typed language.

30.  What is the ABI of the contract?

The Application Binary Interface (ABI) allows anyone writing a smart contract to be able to communicate between a web application written in high-level programming language and the bytecode that is understandable by EVM. 

• ABI defines how the contents of the library are stored inside the file and the program uses the ABI to search through the file and find what it needs.
• These act as function selectors defining the specific methods that can be called to a smart contract for execution.

31. What are the different types of conversions allowed in Solidity?

Solidity allows Implicit and Explicit conversions. 

• Implicit conversion takes place in solidity if it makes sense semantically and no information is lost.
• Explicit conversion can be done in cases when the compiler is not allowing implicit conversion but one is sure of what to do and how to do it. Explicit conversion can be done using casting or constructor-like syntax.

32. How does error handling work in Solidity? 

Solidity provides various functions for error handling where the solidity code is compiled to the byte code and the syntax error check happens at the compile time while the runtime errors are difficult to catch and occur while executing the smart contract.

• When an error occurs the state is reverted back to its original state.
• This undoes all the changes made to the state in the current call to the smart contract.

33. What do revert() and revert(string reason) do?

Both of them are error-handling methods in Solidity.

• revert(): This aborts the execution of the smart contract and reverts the state changes.
• revert(string reason): This aborts the execution of the smart contract and reverts the state changes and provides the explanatory string.

34. What happens if the smart contract does not specify the Solidity version?

Failure to choose the correct version of Solidity while programming the smart contract might lead to the smart contract compilation failure. 

• A contract can be annotated with version pragma that indicates the version that is supported to make code reuse easier.
• There are two ways to use version pragma:
  • “pragma solidity^version”.
  • “pragma solidity version”.

35. Explain the role of the Ethereum Virtual Machine (EVM).

Ethereum Virtual Machine (EVM) serves as the runtime environment for smart contracts running on the Ethereum Blockchain and ensures that smart contracts are executed securely across all nodes in the network.

• The EVM process the EVM bytecode generated from high-level smart contract languages like Solidity.
• EVM is responsible for maintaining the state of the Ethereum blockchain including the contract state. When the contract’s state changes due to the execution of the function then EVM updates the contract’s state.
• EVM uses gas to measure the cost of executing the smart contract. The EVM calculates the total gas used during contract execution.

36. Does EVM understands Solidity?

The EVM does not directly understand Solidity as it understands and executes bytecode. When a transaction is executed EVM reads and processes the corresponding bytecode instructions.

37. What is a proxy contract in Solidity?

A proxy contract in Solidity is a design pattern that employs a secondary contract to act as an intermediary between the users and the main contract. It is often used for storage optimization purposes.

38. What is a staking pool in Solidity?

Staking Pool in Solidity is a smart contract that allows users to pool their cryptocurrency holdings together and participate in various blockchain activities like PoS consensus, etc.

• It helps to maximize potential earnings.
• It also helps to mitigate risks associated with individual staking.

39. List the difference between public and private visibility modifiers in Solidity?

Parameters	Public Visibility Modifier	Private Visibility Modifier
Accessibility	When a state variable, function, or contract is declared as public then it is accessible from any contract including the contract itself, derived contracts, and other external contracts.	When a state variable, function, or contract is declared as private then it is accessible only within the contract in which it is defined.
Accessibility by External Transactions	They can also be called directly by external transactions.	It cannot be accessed from external transactions.
Getter Function	Solidity automatically generates a getter function to allow external access.	No getter functions are generated for private state variables.

40. List different types of libraries in Solidity.

• Deployed Library: Deployed libraries have their own addresses and several other smart contracts can use them.
• Embedded Library: Embedded libraries don’t have their unique address and they are deployed as part of the code of the smart contract that uses them.

41. What is Decentralized Exchange (DEX)?

The Decentralized Exchange (DEX) is a peer-to-peer crypto exchange that operates on a decentralized platform. It does not rely on a single centralized authority to facilitate trade instead it uses several blockchain features like smart contracts and AMM algorithms to enable the trade of crypto assets directly.

• It uses smart contracts to automate trade execution, manage order books, etc.
• DEXs run on a network of nodes, usually powered by blockchain or distributed ledger technology.

42. What is assert and require in Solidity?

Parameters	Assert	Require
Definition	Assert is used to check for code and should never be false.	Require is used to validate inputs and conditions before execution.
Syntax	assert(bool condition)	require(bool condition) require(bool condition, string memory message)
Gas Utility	The assert() function when false, uses up all the remaining gas and reverts all the changes made.	The require() function when false, also reverts back all the changes made to the contract but does refund all the remaining gas fees we offered to pay.

43. What is the formula to calculate Ethereum gas cost?

Gas fees are paid in Ethereum using this formula:

Total gas fees = gasLimit * gasPrice per unit

Here,

gasLimit = maximum amount of gas that is going to spend on a single transaction

gasPrice = minimum gas cost of the execution of a transaction

44. What is the difference between tx.origin and msg.sender?

• tx.origin is the address of the Externally Owner Account (EOA) that originated the transaction.
• msg.sender is the address of the smart contract or EOA that called the currently executing smart contract.

45. What is the largest value a uint256 can store?

uint256 is an unsigned integer that can hold a maximum value of 2^256-1. It requires 32 bytes of storage space.

46. What is a sandwich attack?

The sandwich attack is a form of front-running that targets decentralized finance protocols and services. It leverages the unique characteristics of DeFi to make money by manipulating crypto markets.

• The attacker identifies a potential victim transaction.
• The attacker moves to send a buy transaction for the same asset pair which will fill before the user’s transaction.
• This attacker transaction shifts the liquidity pool composition, resulting in the available exchange rates for subsequent transactions being different from that which was originally shown to other users whose transactions have not yet been executed.
• Once the victim swap completes at the unfavorable exchange rate, the attacker follows up with a sell transaction for the asset pair which locks in their profit.
• This second transaction �kruns’ the victim, completing the sandwich.

47. What is ERC165 used for?

The ERC165 is a standard to publish and detect what interfaces a smart contract implements.

48. What is TWAP?

Time-Wieghted Average Price (TWAP) is an algorithmic strategy that aims to achieve an average execution price close to the time-weighted average price of the user-specified period. TWAP is calculated for executing large trade orders.

49. What is a bonding curve?

Bonding curve is a mathematical concept that is used to describe the relationship between price and token supply.

• Bonding curve contracts sell the tokens to users by calculating the token price in Ether and issuing them after the payment, also buying them and paying with Ether.
• Bonding curve contracts ensure that the price of each token increases as the number of tokens issued increases.

50. What is a reentrancy attack in Solidity?

Reentrancy is a type of attack that can occur in smart contracts that allow untrusted external code to be executed within the contract. This can happen when a smart contract calls an external contract, and the external contract then calls back into the original contract, potentially causing an infinite loop.

• A reentrancy attack is a method of exploiting a vulnerability in a smart contract that allows an attacker to repeatedly call a function in the contract, causing an infinite loop and potentially stealing funds.
• One way to prevent reentrancy attacks is to use a mutex, or mutual exclusion, lock to prevent multiple calls to the same function from occurring at the same time.
• Another way is to use a guard condition, where a flag is set before external function calls and checked after.

Conclusion

In conclusion, Solidity is an trending technology that used by many MNC’s such as Unicsoft, ValueCoders, PixelCrayons. etc. These companies hires Solidity developers frequently. And to get into these companies you need to prepare well about their company specific questions apart from mentioned Solidity interview questions. After doing this, you will definitely succeed into the interviews.

Solidity Interviews Questions – FAQs

Q1. Which companies use Solidity?

Here is the list of companies that frequently hire Solidity Engineer/developer:

1. Unicsoft.
2. ValueCoders.
3. PixelCrayons.
4. OpenXcell
5. SOLULAB
6. dealogic
7. LABRYS
8. S-PRO.
9. Talentica Software.
10. ELEKS

Q2. How much do Solidity programmers make?

According to GeeksforGeeks Experience, The Average Entry-level Salary of Solidity developer ranges from $45,000 – $90,000.

Q3. Is Solidity in demand 2023?

With the continued growth of the blockchain industry, the demand for Solidity developers is likely to increase even further in 2023, as it is the most popular programming language for developing smart contracts on the Ethereum blockchain.

Q4. Is it easy to get a job with Solidity?

There are many job opportunities for Solidity developers. Solidity is one of the highest paid jobs in the blockchain industry.