kamran.cs
class zzz
{
  public static void Main(int i)
  {
  }
}

Compiler Warning
kamran.cs(3,21): warning CS0028: 'zzz.Main(int)' has the wrong signature to be an entry point

Compiler Error
error CS5001: Program 'kamran.exe' does not have an entry point defined

Here, the compiler first displays a warning that Main has not been created with the right parameters. The error, following the warning, proclaims that we have forgotten to create a function called Main. The signature includes the return type only in special cases as entry point.

kamran.cs
class zzz
{
  public static long Main()
  {
    return 0;
  }
}

Compiler Warning
kamran.cs(3,20): warning CS0028: 'zzz.Main()' has the wrong signature to be an entry point

Compiler Error
error CS5001: Program 'kamran.exe' does not have an entry point defined

The signature refers to the parameters given to the function plus the return value. Main in the above program returns 'long', hence we see the error.