What causes "Cannot convert value "{0}" to "{1}"." in dotnet/maui?

Use the literal format the target type's Parse expects (e.g. '.' decimal separator for double).

dotnet/maui · error · BuildException

XC0040

XC0040

Error message

Cannot convert value "{0}" to "{1}".

What it means

Thrown by NodeILExtensions.TryFormat when converting a XAML literal string to a target type (via Parse/TryParse-style func) raises a FormatException. The XAML compiler attempts to convert inline literals (numbers, enums, GUIDs, etc.) to the property's type and reports the offending value and target type.

Source

Thrown at src/Controls/src/Build.Tasks/NodeILExtensions.cs:142

			var typeConverter = bpRef.GetBindablePropertyTypeConverter(context.Cache, module);

			//we're gonna SetValue. if the BP type is Nullable, we only need to convert/box to the non-nullable type? why, because the CSC compiler does it like that
			if (targetTypeRef.ResolveCached(context.Cache).FullName == "System.Nullable`1")
				targetTypeRef = ((GenericInstanceType)targetTypeRef).GenericArguments[0];

			return node.PushConvertedValue(context, targetTypeRef, typeConverter, pushServiceProvider, boxValueTypes,
				unboxValueTypes);
		}

		static T TryFormat<T>(Func<string, T> func, IXmlLineInfo lineInfo, string str)
		{
			try
			{
				return func(str);
			}
			catch (FormatException fex)
			{
				throw new BuildException(BuildExceptionCode.Conversion, lineInfo, fex, str, typeof(T));
			}
		}

		public static IEnumerable<Instruction> PushConvertedValue(this ValueNode node, ILContext context,
			TypeReference targetTypeRef, TypeReference typeConverter, Func<TypeReference[], IEnumerable<Instruction>> pushServiceProvider,
			bool boxValueTypes, bool unboxValueTypes)
		{
			var module = context.Body.Method.Module;
			var knownCompiledTypeConverters = context.Cache.GetKnownCompiledTypeConverters(module);

			var str = (string)node.Value;
			//If the TypeConverter has a ProvideCompiledAttribute that can be resolved, shortcut this
			Type compiledConverterType;
			if (typeConverter?.GetCustomAttribute(context.Cache, module, ("Microsoft.Maui.Controls", "Microsoft.Maui.Controls.Xaml", "ProvideCompiledAttribute"))?.ConstructorArguments?.First().Value is string compiledConverterName
				&& (compiledConverterType = Type.GetType(compiledConverterName)) != null
				|| (typeConverter != null && knownCompiledTypeConverters.TryGetValue(typeConverter, out compiledConverterType)))
			{
				var compiledConverter = Activator.CreateInstance(compiledConverterType);

View on GitHub (pinned to f377ff1c5e)

Solutions

Use the literal format the target type's Parse expects (e.g. '.' decimal separator for double).
For enums, use the exact member name (case-sensitive).
Strip units or use a TypeConverter-aware syntax for unitized values.
If the value must be dynamic, bind it rather than inlining a literal.

Example fix

// before
<Frame Opacity="1,5" />   <!-- comma in invariant culture -->
// after
<Frame Opacity="1.5" />

Defensive patterns

Strategy: try-catch

Validate before calling

// Pre-validate a literal against the target type's Parse (invariant culture)
static bool CanConvert<T>(string literal) {
    try { _ = (T)System.ComponentModel.TypeDescriptor.GetConverter(typeof(T)).ConvertFromString(null, System.Globalization.CultureInfo.InvariantCulture, literal); return true; }
    catch { return false; }
}

Try / catch

// If you author a custom TypeConverter's compiled path, wrap Parse in TryFormat-style guard
try { return func(str); }
catch (FormatException fex) {
    throw new BuildException(BuildExceptionCode.Conversion, lineInfo, fex, str, typeof(T));
}

Prevention

Use the invariant (en-US) literal format: '.' decimals, exact enum names.
Avoid unit suffixes on properties without a TypeConverter for them.
Bind dynamic values instead of inlining literals that need parsing.

When it happens

Trigger: Setting a numeric property to a non-numeric string; an enum property set to an undefined member; a GUID/DateTime property with a malformed literal; locale-specific number separators that Parse rejects.

Common situations: Typing a value like Opacity="1,5" in a comma-decimal locale where '.' is expected; misspelling an enum value; passing a string where a typed converter expects a specific format; unit suffixes like '10px' on a double property.

Related errors

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/917fba8edb63386a. Report an issue: GitHub.