What causes "The exception argument cannot be null." in dotnet/wpf?

Pass the actual caught Exception instance into the constructor.

dotnet/wpf · error · ArgumentNullException

The exception argument cannot be null.

Error message

The exception argument cannot be null.

What it means

The PropertyMappingExceptionEventArgs constructor validates its arguments up front: the exception to report must be non-null because the event args exist solely to carry a mapping failure. Passing null throws ArgumentNullException naming 'exception'.

Solutions

Pass the actual caught Exception instance into the constructor.
Guard before constructing: if (ex != null) new PropertyMappingExceptionEventArgs(ex, prop, value);
Remove code paths that synthesize event args without a real exception.

Example fix

// before
new PropertyMappingExceptionEventArgs(lastError, "Background", value); // lastError may be null
// after
if (lastError != null)
    new PropertyMappingExceptionEventArgs(lastError, "Background", value);

Defensive patterns

Strategy: validation

Validate before calling

if (ex == null) throw new InvalidOperationException("Cannot report a mapping error without an exception");
var args = new PropertyMappingExceptionEventArgs(ex, propertyName, value);

Type guard

bool CanBuildMappingEventArgs(Exception ex, string prop) => ex != null && !string.IsNullOrEmpty(prop);

Try / catch

try { var e = new PropertyMappingExceptionEventArgs(ex, prop, value); }
catch (ArgumentNullException) { log.Error("No exception captured to report"); }

Prevention

Never build event args from a nullable exception field without a null check
Capture the exception inside the catch block and forward it immediately
In tests, always pass a concrete exception instance (e.g. new InvalidOperationException())

When it happens

Trigger: Constructing PropertyMappingExceptionEventArgs(null, "someProp", value) inside a PropertyMappingError handler or test that builds the event args manually; catching and forwarding an exception variable that turned out to be null.

Common situations: Unit tests simulating mapping errors; wrapper code that stores the last exception in a field initialized to null and then builds event args from it.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b1dff31bea8bc0a7. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsFormsIntegration/System/Windows/Integration/PropertyMappingExceptionEventArgs.cs:24

namespace System.Windows.Forms.Integration
{
    /// <summary>
    /// Enables the user to see the property that threw an exception, and to preview or cancel the exception.
    /// </summary>
    public class PropertyMappingExceptionEventArgs : IntegrationExceptionEventArgs 
    {
        private string _propertyName;
        private object _propertyValue;

        /// <summary>
        /// Initializes a new instance of the PropertyMappingExceptionEventArgs class.
        /// </summary>
        public PropertyMappingExceptionEventArgs(Exception exception, string propertyName, object propertyValue) 
            : base(false, exception)
        {
            if (exception == null)
            {
                throw new ArgumentNullException(string.Format(CultureInfo.CurrentCulture, SR.WFI_NullArgument, "exception"));
            }
            if (string.IsNullOrEmpty(propertyName))
            {
                throw new ArgumentNullException(string.Format(CultureInfo.CurrentCulture, SR.WFI_ArgumentNullOrEmpty, "propertyName"));
            }
            _propertyName = propertyName;
            _propertyValue = propertyValue;
        }

        /// <summary>
        /// Identifies the property that was being mapped when the exception occurred.
        /// </summary>
        public string PropertyName
        {
            get
            {
                return _propertyName;
            }

View on GitHub (pinned to 81131a70a4)