unoplatform/uno · error · NotSupportedException

Conversion from type {0} is not supported

Error message

Conversion from type {0} is not supported

What it means

Thrown by XamlTypeTypeConverter.ConvertFrom — the type converter for XamlType always rejects ConvertFrom. It exists to support ConvertTo (XamlType → string) but does not implement any conversion path from arbitrary values (including strings) back to XamlType, so ConvertFrom unconditionally throws NotSupportedException.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeTypeConverter.cs:46

using Uno.Xaml;

namespace Uno.Xaml.Schema
{
	public class XamlTypeTypeConverter : TypeConverter
	{
		public override bool CanConvertFrom (ITypeDescriptorContext context, Type sourceType)
		{
			return sourceType == typeof (string);
		}

		public override bool CanConvertTo (ITypeDescriptorContext context, Type destinationType)
		{
			return destinationType == typeof (string);
		}

		public override Object ConvertFrom (ITypeDescriptorContext context, CultureInfo culture, Object value)
		{
			throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Conversion from type {0} is not supported", value != null ? value.GetType () : null));
		}

		public override object ConvertTo (ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
		{
			if (!CanConvertTo (context, destinationType))
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Conversion to type {0} is not supported", destinationType));

			var vctx = (IValueSerializerContext) context;
			var lookup = vctx != null ? (INamespacePrefixLookup) vctx.GetService (typeof (INamespacePrefixLookup)) : null;
			var xt = value as XamlType;
			if (xt != null && destinationType == typeof (string)) {
				if (lookup != null)
					return new XamlTypeName (xt).ToString (lookup);
				else
					return xt.UnderlyingType != null ? xt.UnderlyingType.ToString () : xt.ToString ();
			}
			else
				return base.ConvertTo (context, culture, value, destinationType); // it seems it still handles not-supported types (such as int).

View on GitHub (pinned to 0418340488)

Solutions

1. Do not call ConvertFrom on XamlTypeTypeConverter — it is intentionally unsupported. Use ConvertTo to go XamlType → string only.
2. If you need string → XamlType, resolve the type via XamlSchemaContext.GetXamlType(string) or XamlTypeName parsing instead.
3. Check CanConvertFrom before calling ConvertFrom — it returns false for all types except... actually it only returns true for typeof(string) but ConvertFrom still throws; treat XamlTypeTypeConverter as convert-to-only.

Example fix

// before
var converter = new XamlTypeTypeConverter();
var xamlType = converter.ConvertFrom(context, culture, "{ns}Name");

// after
var xamlType = schemaContext.GetXamlType(new XamlTypeName("ns", "Name"));

var converter = new XamlTypeTypeConverter();
if (converter.CanConvertFrom(context, value?.GetType() ?? typeof(object)))
    result = converter.ConvertFrom(context, culture, value);
else
    result = ResolveViaSchemaContext(value); // custom path

static bool SupportsConvertFrom(XamlTypeTypeConverter c, Type t) => false; // never supported

try { converter.ConvertFrom(ctx, culture, val); }
catch (NotSupportedException) { /* use schema context resolution instead */ }

When it happens

Trigger: Calling ConvertFrom on a XamlTypeTypeConverter instance with any value type. This can happen via framework plumbing (XAML reader deserialization, TypeDescriptor.GetConverter(typeof(XamlType))) that tries round-trip conversion.

Common situations: A XAML reader or serializer infrastructure attempts bidirectional conversion on a XamlType-valued property and invokes ConvertFrom during deserialization. Misconfigured TypeConverter wiring where a custom adapter expects round-trippable converters.

Related errors

• Conversion to type {0} is not supported
• Cannot cast object '{0}' to string
• instance type '{0}' has no default constructor.
• context
• Value must be non-null string.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/b60aacac55e52509. Report an issue: GitHub.