Assert.Null
Assert.Null and Assert.IsNull test that the specified object is null. The two forms are provided for compatibility with past versions of NUnit and NUnitLite.
Note
From version 4.5.0, using C# 14, you don't need to use the ClassicAssert class, nor the NUnit.Framework.Legacy namespace, but can use the former Assert class. This applies for many of the asserts, but a few still require the use of the ClassicAssert class. These will be fixed in upcoming releases. In the list below, and in the examples, the 4.5.0 syntax will be used. If you use an earlier 4.x version, replace Assert with ClassicAssert and include the NUnit.Framework.Legacy namespace.
Assert.Null(object anObject);
Assert.Null(object anObject, string message, params object[] params);
Assert.IsNull(object anObject);
Assert.IsNull(object anObject, string message, params object[] params);