C# Error CS1107 – A parameter can only have one ‘{0}’ modifier

C# Error

CS1107 – A parameter can only have one ‘{0}’ modifier

Reason for the Error & Solution

A parameter can only have one ‘modifier name’ modifier.

It is an error for parameter modifiers such as this, ref, in, and out to appear more than one time in a parameter definition.

Example

The following example generates CS1107:

// cs1107.cs  
public static class Test
{
    // Extension methods.  
    public static void TestMethod(this this int t) { } // CS1107

}

public class TestTwo
{
    // Regular Instance Method  
    public void TestMethod(ref ref int i) { } // CS1107  

    // Regular Instance Method  
    public void TestMethod(in in double d) { } // CS1107  
}