DependencyProperty double that accepts "Auto" seems to work but throws errors

Question

I want it to be possible to set a height property on a user control with the "Auto" string. ``` public object ContentHeight { get { return GetValue(ContentHeightProperty); } set { SetValue(ContentHeightProperty, value); } } public static readonly DependencyProperty ContentHeightProperty = DependencyProperty.Register("ContentHeight", typeof(object), typeof(UcDataTempl), new PropertyMetadata(new object(), (o, args) => { }, (o, value) => { if (value.Equals("Auto")) return Double.NaN; return value; }), value => true); ``` It seems to be working just fine but I get these errors in the output window: ``` System.Windows.Data Error: 23 : Cannot convert 'System.Object' from type 'Object' to type 'System.Double' for 'en-US' culture with default conversions; consider using Converter property of Binding. NotSupportedException:'System.NotSupportedException: DoubleConverter cannot convert from System.Object. at System.ComponentModel.TypeConverter.GetConvertFromException(Object value) at System.Comp…

Accepted Answer

Here's the implementation of the FrameworkElement.Height property:

```
[TypeConverter(typeof (LengthConverter))]
public double Height
{
  get
  {
    return (double) this.GetValue(FrameworkElement.HeightProperty);
  }
  set
  {
    this.SetValue(FrameworkElement.HeightProperty, (object) value);
  }
}
```

LengthConverter handles conversion of Auto to double.NaN.

So what you need is

```
[TypeConverter(typeof (LengthConverter))]
public double ContentHeight
{
    get { return GetValue(ContentHeightProperty); }
    set { SetValue(ContentHeightProperty, value); }
}
```

DependencyProperty double that accepts "Auto" seems to work but throws errors

Solution

Problem