dotnet/maui · error · ArgumentException

The parameter must be null.

Error message

The parameter must be null.

What it means

Verify.IsNull<T> (where T : class) throws ArgumentException when obj is NOT null. It is the inverse of the usual null check — it asserts that the argument must be null at this point. This is used for state-transition guards where a non-null value indicates the object is in the wrong lifecycle state.

Source

Thrown at src/Compatibility/Core/src/WPF/Microsoft.Windows.Shell/Standard/Verify.cs:139

			if (null == obj)
			{
				Assert.Fail();
				throw new ArgumentNullException(name);
			}
		}

		/// <summary>Verifies that an argument is null.</summary>
		/// <typeparam name="T">Type of the object to validate.  Must be a class.</typeparam>
		/// <param name="obj">The object to validate.</param>
		/// <param name="name">The name of the parameter that will be presented if an exception is thrown.</param>
		[SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
		[DebuggerStepThrough]
		public static void IsNull<T>(T obj, string name) where T : class
		{
			if (null != obj)
			{
				Assert.Fail();
				throw new ArgumentException("The parameter must be null.", name);
			}
		}

		[SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
		[DebuggerStepThrough]
		public static void PropertyIsNotNull<T>(T obj, string name) where T : class
		{
			if (null == obj)
			{
				Assert.Fail();
				throw new InvalidOperationException(string.Format(CultureInfo.InvariantCulture, "The property {0} cannot be null at this time.", name));
			}
		}

		[SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
		[DebuggerStepThrough]
		public static void PropertyIsNull<T>(T obj, string name) where T : class
		{

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Ensure the referenced object is set to null before calling the method that requires null.
2. Check the lifecycle: if the object must be null, verify it has been reset/cleared first.
3. Restructure to avoid double-initialization; guard the calling code against re-entry.

Example fix

// before
window.Initialize(); // internal reference already set

// after
window.Reset(); // sets internal reference to null
window.Initialize();

// Pre-check: the API requires the reference to be null
if (obj != null)
{
    throw new ArgumentException("Reference must be null at this point.", nameof(obj));
}
SomeApi(obj);

When it happens

Trigger: Calling a method guarded by Verify.IsNull(obj, name) when obj has already been assigned a non-null reference. The method expects the value to be unset/cleared at the time of the call.

Common situations: Calling an Initialize/Set method twice without first clearing the previous value; a dispose-then-reset path where the field was not nulled after disposal; lifecycle ordering where the caller advanced the object state out of order.

Related errors

• The parameter cannot be either null or empty.
• The parameter cannot be either null or empty or consist only
• The parameter must not be the default value.
• The property {0} must be null at this time.
• The URI must be absolute.

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/50f3492cb883bc3d. Report an issue: GitHub.