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.

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

• No file exists at
• The integer value must be bounded with
• The parameter cannot be either null or empty.
• The parameter cannot be either null or empty or consist…
• The parameter must implement interface

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

Appendix: 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
		{

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