dotnet/maui · error · NotSupportedException

throw new NotSupportedException();

Error message

throw new NotSupportedException();

What it means

FileImageSourceConverter.ConvertTo throws a bare NotSupportedException (no message) when the value passed in is not a FileImageSource. The converter only serializes a FileImageSource back to its File string; any other type is unsupported. The lack of a message makes diagnostics harder.

Source

Thrown at src/Controls/src/Core/FileImageSourceConverter.cs:28

		public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
			=> sourceType == typeof(string);

		public override bool CanConvertTo(ITypeDescriptorContext? context, Type? destinationType)
			=> destinationType == typeof(string);

		public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
		{
			var strValue = value?.ToString();
			if (strValue != null)
				return (FileImageSource)ImageSource.FromFile(strValue);

			throw new InvalidOperationException(string.Format("Cannot convert \"{0}\" into {1}", strValue, typeof(FileImageSource)));
		}

		public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
		{
			if (value is not FileImageSource fis)
				throw new NotSupportedException();
			return fis.File;
		}
	}
}

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Route conversion through the generic ImageSourceConverter or ImageTypeConverter that handles all ImageSource kinds.
2. Type-check before calling: `if (value is FileImageSource fis) ... else use appropriate converter`.
3. Avoid ConvertTo for design-time serialization of polymorphic ImageSource values.

Example fix

// before
var path = new FileImageSourceConverter().ConvertTo(myUriImageSource, typeof(string));

// after
if (value is FileImageSource fis)
    return fis.File;
throw new NotSupportedException($"Expected FileImageSource, got {value?.GetType()}.");

if (value is not FileImageSource) throw new NotSupportedException();

static bool IsFileImageSource(object v) => v is FileImageSource;

try { return converter.ConvertTo(value, typeof(string)); } catch (NotSupportedException) { /* route to correct ImageSource converter */ }

When it happens

Trigger: Calling ConvertTo with a StreamImageSource, FontImageSource, UriImageSource, or a non-ImageSource object. XAML round-tripping or serialization pipelines that route mixed ImageSource kinds through one converter.

Common situations: Switching an Image source from a file to a URI/font image while a converter-based serializer or property grid still routes through FileImageSourceConverter.

Related errors

• Cannot convert "{0}" into {1}
• throw new NotSupportedException();
• XC0040
• Cannot convert "{strValue}" into {typeof(FlowDirection)}
• No test assembly found.

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/f4e06e73fdd87a42. Report an issue: GitHub.