dotnet/wpf · error · InvalidOperationException

The property must be null at this time.

Error message

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

What it means

Verify.PropertyIsNull<T> is the inverse assertion for properties: it demands that the property currently be null and throws InvalidOperationException if a value is still present. It enforces lifecycle state — typically that a resource was released or not yet created.

Solutions

1. Null out the property (after disposing) before re-running the guarded code path.
2. Add proper Dispose/cleanup so the property is cleared on teardown.
3. If re-initialization is legal, relax or remove this assertion and use an explicit state flag instead.
4. Check the interpolated property name in the message to find the stale reference.

Example fix

// before
Verify.PropertyIsNull(this.Handler, nameof(this.Handler)); // still set
this.Handler = CreateHandler();
// after
(this.Handler as IDisposable)?.Dispose();
this.Handler = null;
Verify.PropertyIsNull(this.Handler, nameof(this.Handler));
this.Handler = CreateHandler();

if (this.Handler != null)
    throw new InvalidOperationException("Handler must be released before re-initialization");
Verify.PropertyIsNull(this.Handler, nameof(this.Handler));

static bool IsCleared<T>(T prop) where T : class => prop is null;

try {
    Verify.PropertyIsNull(this.Handler, nameof(this.Handler));
} catch (InvalidOperationException ex) {
    (this.Handler as IDisposable)?.Dispose();
    this.Handler = null;
}

When it happens

Trigger: Calling Verify.PropertyIsNull(obj, name) while the property still holds a reference — e.g. re-initializing or disposing a component that asserts a previous instance was cleared, but cleanup didn't null it.

Common situations: Double initialization of a singleton property; Dispose patterns where the field reset was removed; re-entering setup code after a partial failure.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• The property cannot be null at this time.
• CurrentFixedPageWriter uninitialized
• FixedPageReader
• SR.AnnotationServiceNotEnabled
• SR.ApplicationShuttingDown

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0af727403a675f9c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Standard/Verify.cs:147

                throw new ArgumentException("The parameter must be null.", name);
            }
        }
        
        [DebuggerStepThrough]
        public static void PropertyIsNotNull<T>(T obj, string name) where T : class
        {
            if (null == obj)
            {
                throw new InvalidOperationException($"The property {name} cannot be null at this time.");
            }
        }
        
        [DebuggerStepThrough]
        public static void PropertyIsNull<T>(T obj, string name) where T : class
        {
            if (null != obj)
            {
                throw new InvalidOperationException($"The property {name} must be null at this time.");
            }
        }

        /// <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>    
        [DebuggerStepThrough]
        public static void IsTrue(bool statement, string name)
        {
            if (!statement)
            {
                throw new ArgumentException("", name);
            }
        }

        /// <summary>

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