dotnet/maui · error · InvalidOperationException

The property {0} must be null at this time.

Error message

The property {0} must be null at this time.

What it means

Verify.PropertyIsNull<T> (where T : class) throws InvalidOperationException when the property value is NOT null. This is the inverse state guard — it asserts a property must be unset at this point. Used in lifecycle transitions where a non-null property means the object has already been configured and should not be reconfigured.

Source

Thrown at src/Compatibility/Core/src/WPF/Microsoft.Windows.Shell/Standard/Verify.cs:161

		[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
		{
			if (null != obj)
			{
				Assert.Fail();
				throw new InvalidOperationException(string.Format(CultureInfo.InvariantCulture, "The property {0} must be null at this time.", name));
			}
		}

		/// <summary>
		/// Verifies the specified statement is true.  Throws an ArgumentException if it's not.
		/// </summary>
		/// <param name="statement">The statement to be verified as true.</param>
		/// <param name="name">Name of the parameter to include in the ArgumentException.</param>
		/// <param name="message">The message to include in the ArgumentException.</param>
		[SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
		[DebuggerStepThrough]
		public static void IsTrue(bool statement, string name, string message = null)
		{
			if (!statement)
			{
				Assert.Fail();
				throw new ArgumentException(message ?? "", name);
			}

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Reset the property to null before re-invoking the method that requires it unset.
2. Guard against re-entry at the call site (if (_prop != null) return or throw).
3. Restructure the lifecycle so the property is set exactly once.

Example fix

// before
manager.Configure(); // already configured

// after
manager.Reset(); // clears the property to null
manager.Configure();

// Pre-check: property must be null before the operation
if (obj.SomeProperty != null)
{
    throw new InvalidOperationException("SomeProperty must be null before reconfiguration. Call Reset() first.");
}
obj.Reconfigure();

try
{
    obj.Reconfigure();
}
catch (InvalidOperationException ex) when (ex.Message.Contains("must be null"))
{
    obj.Reset();
    obj.Reconfigure();
}

When it happens

Trigger: Calling a method guarded by Verify.PropertyIsNull(_prop, "Prop") when _prop has already been assigned. The guard fires because the operation expects the property to be cleared first.

Common situations: Double-configuration of an object (calling Configure twice); re-initialization without reset; a setter that should only fire once but is invoked again on a second binding pass.

Related errors

• The property {0} cannot be null at this time.
• The parameter must be null.
• 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.

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/81c294570995605d. Report an issue: GitHub.