dotnet/wpf · error · InvalidEnumArgumentException

InvalidEnumArgumentException("button", (int)button…

Error message

InvalidEnumArgumentException("button", (int)button, typeof(MouseButton))

What it means

MouseButton.Validate throws System.ComponentModel.InvalidEnumArgumentException when the button argument is not a defined MouseButton value (Left, Right, Middle, XButton1, XButton2). WPF validates public enum parameters to catch corrupted or miscomputed values early.

Solutions

Validate the value against Enum.IsDefined(typeof(MouseButton), value) before passing it
Map external button codes to MouseButton explicitly with a switch
Fix the source that produces the out-of-range value

Example fix

// before
MouseButton b = (MouseButton)rawValue;
MouseButton.Validate(b); // throws
// after
if (!Enum.IsDefined(typeof(MouseButton), rawValue))
{
    rawValue = (int)MouseButton.Left; // or map explicitly
}
MouseButton b = (MouseButton)rawValue;

Defensive patterns

Strategy: validation

Validate before calling

static bool IsValidMouseButton(MouseButton b) =>
    b == MouseButton.Left || b == MouseButton.Right || b == MouseButton.Middle ||
    b == MouseButton.XButton1 || b == MouseButton.XButton2;

Type guard

static bool TryParseMouseButton(int raw, out MouseButton button)
{ if (raw >= 0 && raw <= 4) { button = (MouseButton)raw; return true; } button = MouseButton.Left; return false; }

Try / catch

try { MouseButton.Validate(button); }
catch (InvalidEnumArgumentException) { button = MouseButton.Left; }

Prevention

Map Win32/external button codes explicitly instead of casting ints
Use Enum.IsDefined before any enum cast
Never combine MouseButton values with bitwise operators

When it happens

Trigger: Passing an int cast to MouseButton that is outside 0-4, e.g. from P/Invoke data, deserialized state, or bit-flag arithmetic on button values.

Common situations: Interoperating with Win32 mouse messages where button codes differ, or computing a button from config/serialization that holds an out-of-range integer.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c2cba1762260fbb9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/MouseButton.cs:75

        /// <summary>
        ///     Ensures MouseButton is set to a valid value.
        /// </summary>
        /// <remarks>
        ///     There is a proscription against using Enum.IsDefined().  (it is slow)
        ///     So we manually validate using a switch statement.
        /// </remarks>
        internal static void Validate(MouseButton button)
        {
            switch(button)
            {
                case MouseButton.Left:
                case MouseButton.Middle:
                case MouseButton.Right:
                case MouseButton.XButton1:
                case MouseButton.XButton2:
                    break;
                default:
                    throw new  System.ComponentModel.InvalidEnumArgumentException("button", (int)button, typeof(MouseButton));
            }
        }
}
}

View on GitHub (pinned to 81131a70a4)