linux-packaging-mono/mcs/tests/test-924.cs

using System;

class MainClass
{
	public static int Main ()
	{
		DataFrame df1 = new DataFrame ();
		DataFrame df2 = new DataFrame ();

		if (df1 != null) 
		{
			return 1;
		}

		return 0;
	}

	public class DataFrame
	{
		public static bool operator ==(DataFrame df1, DataFrame df2)
		{
			return df1 is DataFrame;
		}

		public static bool operator !=(DataFrame df1, DataFrame df2)
		{
			return !(df1 == df2);
		}
	}
}