C# | String.ToUpperInvariant Method

String.ToUpperInvariant Method is used to get a copy of this String object converted to uppercase using the casing rules of the invariant culture. Here “invariant culture” represents a culture that is culture-insensitive.
Syntax:

public string ToUpperInvariant ();

Return Value: The return type of this method is System.String. This method will return a string which is the uppercase equivalent of the current string.
Below given are some examples to understand the implementation in a better way: 
Example 1:

CSharp

// C# program to illustrate
// ToUpperInvariant() method
using System;

public class GFG {

    // Main method
    static public void Main()
    {

        // variables
        string strA = "WelCome tO GeeKSfOrGeeKs";
        string strB;

        // Convert strA into lowercase
        // using ToLowerInvariant() method
        strB = strA.ToUpperInvariant();

        // Display string before ToUpperInvariant() method
        Console.WriteLine("String before ToUpperInvariant:");
        Console.WriteLine(strA);
        Console.WriteLine();

        // Display string after ToUpperInvariant() method
        Console.WriteLine("String after ToUpperInvariant:");
        Console.WriteLine(strB);
    }
}

Output:

String before ToUpperInvariant:
WelCome tO GeeKSfOrGeeKs

String after ToUpperInvariant:
WELCOME TO GEEKSFORGEEKS

Example 2:

CSharp

// C# program to illustrate
// ToUpperInvariant() Method
using System;

public class GFG {

    // Main method
    static public void Main()
    {

        // Calling function
        Convert("GEeks");
        Convert("geeks");
        Convert("GEEKS");
    }

    static void Convert(String value)
    {

        // Display  strings
        Console.WriteLine("string 1:  {0}", value);

        // Convert string into Uppercase
        // using ToUpperInvariant() method
        value = value.ToUpperInvariant();

        // Display the Lowercase strings
        Console.WriteLine("string 2:  {0}", value);
    }
}

Output:

string 1:  GEeks
string 2:  GEEKS
string 1:  geeks
string 2:  GEEKS
string 1:  GEEKS
string 2:  GEEKS

Note:

• The invariant culture represents a culture that is culture-insensitive. It is associated with the English language but not with a specific country or region.
• ToUpperInvariant() method does not modify the value of the current instance. Instead, it returns a new string in which all characters in the current instance are converted to uppercase.
• This method can’t be overloaded if you try to overload this method, it will give you compile time error.

Reference: https://docs.microsoft.com/en-us/dotnet/api/system.string.toupperinvariant?view=netframework-4.7.2