unoplatform/uno · error · XamlXmlWriterException

Value type is '{0}' but it must be either string or any type

Error message

Value type is '{0}' but it must be either string or any type that is convertible to string indicated by TypeConverterAttribute.

What it means

The internal value-to-string converter in XamlWriterInternalBase throws XamlXmlWriterException when it must serialize a value to text but the value is neither a string nor a type with a ValueSerializer (via the member's ValueSerializer or the type's ValueSerializer). The converter checks for string/empty-string fast paths, then asks the XamlType for a ValueSerializer; if none exists it cannot emit a textual representation and fails.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlWriterInternalBase.cs:252

		protected abstract void OnWriteValue (object value);

		protected abstract void OnWriteNamespace (NamespaceDeclaration nd);
		
		protected string GetValueString (XamlMember xm, object value)
		{
			// change XamlXmlReader too if we change here.
			if ((value as string) == String.Empty) // FIXME: there could be some escape syntax.
				return "\"\"";
			if (value is string)
				return (string) value;

			var xt = value == null ? XamlLanguage.Null : sctx.GetXamlType (value.GetType ());
			var vs = xm.ValueSerializer ?? xt.ValueSerializer;
			if (vs != null)
				return vs.ConverterInstance.ConvertToString (value, service_provider);
			else
				throw new XamlXmlWriterException (String.Format (CultureInfo.InvariantCulture, "Value type is '{0}' but it must be either string or any type that is convertible to string indicated by TypeConverterAttribute.", value != null ? value.GetType () : null));
		}
	}
}

View on GitHub (pinned to 0418340488)

Solutions

1. Add a TypeConverter (and corresponding ValueSerializer) to the value's type so the writer can convert it to/from string.
2. Restructure the object graph so the property holds a string or a type that already has a ValueSerializer (e.g. primitives, DateTime with the XAML serializer).
3. If the value is genuinely complex, emit it as a nested object rather than a text value.
4. Use a custom XamlXmlWriter subclass overriding value handling if you need bespoke serialization.

Example fix

// before
public class Color { public int R,G,B; } // no converter
// obj.MyColor = new Color {...};  -> save fails

// after
[TypeConverter(typeof(ColorConverter))]
public class Color { public int R,G,B; }
public class ColorConverter : TypeConverter {
    public override bool CanConvertTo(ITypeDescriptorContext c, Type t) => t == typeof(string);
    public override object ConvertTo(ITypeDescriptorContext c, CultureInfo ci, object v, Type t) => /* format */;
}

var xt = sctx.GetXamlType(value.GetType());
if (!(value is string) && xt.ValueSerializer == null && member?.ValueSerializer == null)
    throw new InvalidOperationException($"Cannot serialize {value.GetType()} as text; add a ValueSerializer/TypeConverter.");

static bool IsSerializableAsText(object? v, XamlMember? m, XamlSchemaContext sctx) {
    if (v is string || v is null) return true;
    var xt = sctx.GetXamlType(v.GetType());
    return (m?.ValueSerializer ?? xt.ValueSerializer) != null;
}

try { XamlServices.Save(writer, instance); }
catch (XamlXmlWriterException ex) when (ex.Message.Contains("convertible to string")) {
    // identify the offending value type in the message, then attach a TypeConverter
}

When it happens

Trigger: Saving an object graph where a property holds a complex type with no associated TypeConverter/ValueSerializer — e.g. a custom class assigned to a content property that the writer must render as text but cannot convert.

Common situations: Serializing domain objects that lack [TypeConverter] or ValueSerializer attributes; persisting object graphs that include non-serializable nested types in text positions; round-tripping XAML where a value type changed to a non-convertible type.

Related errors

• Cannot cast object '{0}' to string
• context
• Value must be non-null string.
• descriptor
• type

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/b4b2930f1cac1ba1. Report an issue: GitHub.