MahApps/MahApps.Metro · error · InvalidCastException

Cannot convert the given input to a valid color

Error message

Cannot convert the given input to a valid color

What it means

ColorPickerBase.OnColorNamePropertyChanged fires when the bound ColorName string changes. It asks the configured ColorHelper to parse the string; when parsing yields no color (the helper returns null), the callback throws InvalidCastException. This rejects any string the helper cannot map to a known color name or hex value.

Source

Thrown at src/MahApps.Metro/Controls/ColorPicker/ColorPickerBase.cs:139

                    {
                        colorPicker.SetCurrentValue(SelectedColorProperty, color);
                    }
                    else // if the color stayed the same we still have to update the displayed name
                    {
                        colorPicker.ColorIsUpdating = true;
                        try
                        {
                            colorPicker.SetCurrentValue(ColorNameProperty, (colorPicker.ColorHelper ?? ColorHelper.DefaultInstance).GetColorName(color, colorPicker.ColorNamesDictionary, colorPicker.IsAlphaChannelVisible));
                        }
                        finally
                        {
                            colorPicker.ColorIsUpdating = false;
                        }
                    }
                }
                else
                {
                    throw new InvalidCastException("Cannot convert the given input to a valid color");
                }
            }
        }

        /// <summary>
        /// Gets or sets the name of the <see cref="SelectedColor"/>.
        /// </summary>
        public string? ColorName
        {
            get => (string?)this.GetValue(ColorNameProperty);
            set => this.SetValue(ColorNameProperty, value);
        }

        /// <summary>Identifies the <see cref="ColorNamesDictionary"/> dependency property.</summary>
        public static readonly DependencyProperty ColorNamesDictionaryProperty
            = DependencyProperty.Register(nameof(ColorNamesDictionary),
                                          typeof(Dictionary<Color, string>),
                                          typeof(ColorPickerBase),

View on GitHub (pinned to 72099e310b)

Solutions

1. Validate the input string before assigning to ColorName — ensure it parses to a Color via the same ColorHelper/ColorNamesDictionary first.
2. Provide a ColorNamesDictionary that includes the localized/custom names you accept, or restrict the input to known names.
3. Set IsAlphaChannelVisible to match the hex format you feed in (e.g. #AARRGGBB when alpha is visible).
4. Supply a custom ColorHelper that can parse your color name format instead of returning null.

Example fix

// before: unknown name throws inside the DP callback
ColorNameTextBox.TextChanged += (s,e) => vm.ColorName = ColorNameTextBox.Text;
// after: validate first
var helper = ColorHelper.DefaultInstance;
var parsed = helper.ColorFromString(input, vm.ColorNamesDictionary);
if (parsed is not null) vm.ColorName = input;
else vm.ColorError = $"'{input}' is not a valid color";

// Validate the string can be parsed by the same helper before assigning ColorName
var helper = colorPicker.ColorHelper ?? ColorHelper.DefaultInstance;
var color = helper.ColorFromString(input, colorPicker.ColorNamesDictionary);
if (color is null) {
    // show user-facing error, do NOT assign to ColorName
    return;
}
colorPicker.ColorName = input;

try {
    colorPicker.ColorName = input;
} catch (InvalidCastException) {
    // input not parseable; keep previous color, surface a user message
}

When it happens

Trigger: ColorName is set (via binding or directly) to a value that ColorHelper.ColorFromString cannot parse — neither a recognized name nor a valid hex string — so the helper returns null and the callback throws InvalidCastException during the dependency-property change notification.

Common situations: User types an unrecognized color name in a bound TextBox. Binding feeds a localized name not present in ColorNamesDictionary. A hex string with the wrong format/length (#RGB vs #ARGB) for the configured IsAlphaChannelVisible. A custom ColorHelper that returns null for valid inputs.

Related errors

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AI-assisted analysis of MahApps/MahApps.Metro@72099e310b (2026-08-13). Data as JSON: /api/errors/3762b763fbd4c38e. Report an issue: GitHub.