What causes "The property {name} cannot be null at this time." in dotnet/wpf?

Initialize the property in the constructor or via proper initialization before the guarded call.

dotnet/wpf · error · InvalidOperationException

The property cannot be null at this time.

Error message

The property {name} cannot be null at this time.

What it means

Verify.PropertyIsNotNull<T> asserts that an instance property is initialized, throwing InvalidOperationException (not ArgumentException, since this is object-state validation, not argument validation) when the property is null. The message names the offending property via string interpolation.

Solutions

Initialize the property in the constructor or via proper initialization before the guarded call.
Delay the operation until initialization is complete (e.g. wait for Loaded event in WPF).
Use lazy initialization or a factory to guarantee non-null.
Check the interpolated property name in the message to find which property needs initialization.

Example fix

// before
Verify.PropertyIsNotNull(this.Document, nameof(this.Document));
Document.DoWork();
// after
if (this.Document == null) this.Document = new Document();
Verify.PropertyIsNotNull(this.Document, nameof(this.Document));

Defensive patterns

Strategy: validation

Validate before calling

if (this.Document == null)
    throw new InvalidOperationException("Initialize Document before use");
Verify.PropertyIsNotNull(this.Document, nameof(this.Document));

Type guard

static bool IsInitialized<T>(T prop) where T : class => prop is not null;

Try / catch

try {
    Verify.PropertyIsNotNull(this.Document, nameof(this.Document));
} catch (InvalidOperationException ex) {
    log.Error(ex.Message);
    InitializeDocument();
}

Prevention

Initialize all required properties in constructors or Initialize methods.
In WPF, defer property access until after Loaded/template application.
Use lazy<T> for expensive required properties.

When it happens

Trigger: Calling Verify.PropertyIsNotNull(obj, name) where obj is null — e.g. asserting this.SomeService != null before use when the property was never assigned (initializer not run, constructor skipped, or XAML/load path not executed yet).

Common situations: Accessing a WPF component property before template/load initialization completes; using a class before its constructor finished; lazy property that was expected to be set by a prior step.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b8534dbf86163f8e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Standard/Verify.cs:138

        /// <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>   
        [DebuggerStepThrough]
        public static void IsNull<T>(T obj, string name) where T : class
        {
            if (null != obj)
            {
                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]

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