In C# | Identifiers programming languages, identifiers are used for identification purposes. Or in other words, identifiers are the user-defined name of the program components. In C#, an identifier can be a class name, method name, variable name, or label. 

Example: 

public class GFG {
    static public void Main () 
    {
          int x;
    }
}

Here the total number of identifiers present in the above example is 3 and the names of these identifiers are: 

• GFG: Name of the class
• Main: Method name
• x: Variable name

Rules for defining identifiers in C#:

There are certain valid rules for defining a valid C# identifier. These rules should be followed, otherwise, we will get a compile-time error. 

• The only allowed characters for identifiers are all alphanumeric characters([A-Z], [a-z], [0-9]), ‘_‘ (underscore). For example “tech@” is not a valid C# identifier as it contain ‘@’ – special character.
• Identifiers should not start with digits([0-9]). For example “123tech” is not valid in the C# identifier.
• Identifiers should not contain white spaces.
   
• Identifiers are not allowed to use as keywords unless they include @ as a prefix. For example, @as is a valid identifier, but “as” is not because it is a keyword.
• C# identifiers allow Unicode Characters.
• C# identifiers are case-sensitive.
• C# identifiers cannot contain more than 512 characters.
• Identifiers do not contain two consecutive underscores in their name because such types of identifiers are used for the implementation.

Example:

// Simple C# program to illustrate identifiers
using System;
 
class GFG {
 
    // Main Method
    static public void Main()
    {
 
        // variable
        int a = 10;
        int b = 39;
        int c;
 
        // simple addition
        c = a + b;
        Console.WriteLine("The sum of two number is: {0}", c);
    }
}

Output:

The sum of two number is: 49

Below table shows the identifiers and keywords present in the above example: