unoplatform/uno · error · NotSupportedException

Conversion to type {0} is not supported

Error message

Conversion to type {0} is not supported

What it means

Thrown by XamlTypeTypeConverter.ConvertTo when CanConvertTo returns false for the requested destinationType. The converter only supports converting XamlType to typeof(string); any other destination type (or a non-XamlType source value that falls through to base.ConvertTo) triggers NotSupportedException.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeTypeConverter.cs:52

		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. Only request conversion to typeof(string) from XamlTypeTypeConverter.
2. Check CanConvertTo(context, destinationType) before calling ConvertTo and handle false gracefully.
3. If the value is not a XamlType, use a TypeConverter appropriate to the value's actual type.

Example fix

// before
string name = (string)converter.ConvertTo(ctx, culture, xamlType, typeof(string)); // OK
int bad = (int)converter.ConvertTo(ctx, culture, xamlType, typeof(int)); // throws

// after
if (converter.CanConvertTo(ctx, typeof(string)))
    name = (string)converter.ConvertTo(ctx, culture, xamlType, typeof(string));

if (converter.CanConvertTo(context, destinationType))
    return converter.ConvertTo(context, culture, value, destinationType);

static bool CanConvert(XamlTypeTypeConverter c, ITypeDescriptorContext ctx, Type dest)
    => c.CanConvertTo(ctx, dest);

try { return converter.ConvertTo(ctx, culture, value, destType); }
catch (NotSupportedException) { /* handle unsupported target type */ }

When it happens

Trigger: Calling ConvertTo(context, culture, value, typeof(int)) or any destinationType other than string. Or calling ConvertTo with a value that is not a XamlType and a destinationType that base.ConvertTo also cannot handle.

Common situations: Generic XAML serialization pipeline that enumerates target types. Misconfigured binding or markup-extension code that requests conversion to an incompatible type. Code that assumes a TypeConverter can round-trip to any Type.

Related errors

• Conversion from 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/d6e0e438799c1006. Report an issue: GitHub.