dotnet/wpf · error · NotSupportedException

SR.Converter_ConvertFromNotSupported

Error message

SR.Converter_ConvertFromNotSupported

What it means

KeySplineConverter.ConvertFrom throws NotSupportedException (SR.Converter_ConvertFromNotSupported) when the value argument is null, because the type converter only supports converting string (and KeySpline) input to a KeySpline. Converting from null is explicitly unsupported rather than treated as an empty spline.

Solutions

1. Check for null before calling ConvertFrom and substitute a default KeySpline (e.g. new KeySpline(0,0,1,1)).
2. In XAML, supply a non-null value or a default in markup rather than relying on the converter.
3. Use ConvertFromString only for genuine strings; handle null at the caller level.
4. Catch NotSupportedException and fall back to a default spline if null input is expected in your pipeline.

Example fix

// before
var spline = (KeySpline)converter.ConvertFrom(context, culture, possiblyNull);
// after
var spline = possiblyNull == null
    ? new KeySpline(new Point(0, 0), new Point(1, 1))
    : (KeySpline)converter.ConvertFrom(context, culture, possiblyNull);

if (value == null)
    return new KeySpline(new Point(0, 0), new Point(1, 1)); // default spline
var spline = (KeySpline)converter.ConvertFrom(context, culture, value);

static bool IsConvertibleValue(object v) => v is string or KeySpline;

try
{
    spline = (KeySpline)converter.ConvertFrom(context, culture, value);
}
catch (NotSupportedException)
{
    spline = new KeySpline(new Point(0, 0), new Point(1, 1));
}

When it happens

Trigger: TypeDescriptor/Convert.ChangeType style pipelines calling converter.ConvertFrom(context, culture, null), e.g. during XAML or data-binding value conversion when the source value is null.

Common situations: XAML attribute bound to a null resource; deserializers invoking type converters for missing fields; code calling KeySplineConverter directly with a null input while assuming default-value semantics.

Related errors

• ArgumentNullException (nameof(targetObject))
• Buffer address passed to GetText cannot be NULL.
• Cannot convert from type.
• Cannot convert from type.
• Cannot convert to type.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/88272a76a20a945b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Animation/KeySplineConverter.cs:67

            else
            {
                return false;
            }
        }

        /// <summary>
        /// ConvertFrom
        /// </summary>
        public override object ConvertFrom(
            ITypeDescriptorContext context, 
            CultureInfo cultureInfo, 
            object value)
        {
            string stringValue = value as string;

            if (value == null)
            {
                throw new NotSupportedException(SR.Converter_ConvertFromNotSupported);
            }

            TokenizerHelper th = new TokenizerHelper(stringValue, cultureInfo);

            return new KeySpline(
                Convert.ToDouble(th.NextTokenRequired(), cultureInfo),
                Convert.ToDouble(th.NextTokenRequired(), cultureInfo),
                Convert.ToDouble(th.NextTokenRequired(), cultureInfo),
                Convert.ToDouble(th.NextTokenRequired(), cultureInfo));
        }

        /// <summary>
        /// TypeConverter method implementation.
        /// </summary>
        /// <param name="context">ITypeDescriptorContext</param>
        /// <param name="cultureInfo">current culture (see CLR specs), null is a valid value</param>
        /// <param name="value">value to convert from</param>
        /// <param name="destinationType">Type to convert to</param>

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