unoplatform/uno · error · XamlObjectWriterException

Could not convert object '{0}' (of type {1}) to {2}:

Error message

Could not convert object '{0}' (of type {1}) to {2}: 

What it means

A wrapper thrown by GetCorrectlyTypedValue when coercing a value to the target type/member throws any exception that is not already a XamlObjectWriterException. The inner exception holds the real conversion error, and its message is appended after the colon. The text 'Could not convert object ...' is the generic framing.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlObjectWriter.cs:500

				if (xt.IsDictionary)
					mt.Invoker.AddToDictionary (parent, GetCorrectlyTypedValue (null, xt.KeyType, keyObj), GetCorrectlyTypedValue (null, xt.ItemType, obj));
				else // collection. Note that state.Type isn't usable for PositionalParameters to identify collection kind.
					mt.Invoker.AddToCollection (parent, GetCorrectlyTypedValue (null, xt.ItemType, obj));
				return true;
			}
			else
				return false;
		}

		object GetCorrectlyTypedValue (XamlMember xm, XamlType xt, object value)
		{
			try {
				return DoGetCorrectlyTypedValue (xm, xt, value);
			} catch (XamlObjectWriterException) {
				throw;
			} catch (Exception ex) {
				// For + ex.Message, the runtime should print InnerException message like .NET does.
				throw new XamlObjectWriterException (String.Format (CultureInfo.InvariantCulture, "Could not convert object \'{0}' (of type {1}) to {2}: ", value, value != null ? (object) value.GetType () : "(null)", xt)  + ex.Message, ex);
			}
		}

		// It expects that it is not invoked when there is no value to 
		// assign.
		// When it is passed null, then it returns a default instance.
		// For example, passing null as Int32 results in 0.
		// But do not immediately try to instantiate with the type, since the type might be abstract.
		object DoGetCorrectlyTypedValue (XamlMember xm, XamlType xt, object value)
		{
			if (value == null) {
				if (xt.IsContentValue (service_provider)) // it is for collection/dictionary key and item
					return null;
				else
					return xt.IsNullable ? null : xt.Invoker.CreateInstance (Array.Empty<object>());
			}
			if (xt == null)
				return value;

View on GitHub (pinned to 0418340488)

Solutions

1. Read the InnerException message (appended after the colon) to find the real conversion failure.
2. Correct the source value so the type converter accepts it.
3. Verify the expected TypeConverter is registered for the target type and is functioning.

Example fix

<!-- before -->
<Slider Value="abc" />
<!-- after -->
<Slider Value="42" />

try { /* load/write XAML */ }
catch (XamlObjectWriterException ex) when (ex.Message.Contains("Could not convert")) {
    // ex.InnerException has the converter's failure; fix the source value or the TypeConverter.
    throw new InvalidOperationException("Type conversion failed", ex.InnerException);
}

When it happens

Trigger: A registered TypeConverter throws during ConvertFrom — e.g. Int32Converter receiving 'abc', a custom converter rejecting input, or a culture-sensitive parse failing.

Common situations: Malformed string values in XAML attributes; custom TypeConverter bugs; locale/culture parsing differences; null where a value type was expected.

Related errors

• An error occurred on getting provided value
• Value '{0}' (of type {1}) is not of or convertible to type {
• Member '{0}' is already written to current type '{1}'
• RootObjectInstance type '{0}' is not assignable to '{1}'
• The value for '{0}' property is null

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/91d8be833c3c9c37. Report an issue: GitHub.