dotnet/maui · error · BuildException

XC0004

XC0004

Error message

Missing default constructor for "{0}".

What it means

Thrown when the type is a non-value-type (and not the special-cased Microsoft.Maui.Graphics.Color), no parameterless public constructor exists, no factory method applies, and no parameterized [Parameter] constructor was satisfied. The XAML compiler cannot instantiate the object without one of these.

Source

Thrown at src/Controls/src/Build.Tasks/CreateObjectVisitor.cs:184

			if (parameterizedCtorInfo != null && ctorInfo == null)
				//there was a parameterized ctor, we didn't use it
				throw new BuildException(BuildExceptionCode.PropertyMissing, node, null, missingCtorParameter, typedef.FullName);
			var ctorinforef = ctorInfo?.ResolveGenericParameters(typeref, Module);
			var factorymethodinforef = factoryMethodInfo?.ResolveGenericParameters(typeref, Module);
			var implicitOperatorref = typedef.Methods.FirstOrDefault(md =>
				md.IsPublic &&
				md.IsStatic &&
				md.IsSpecialName &&
				md.Name == "op_Implicit" && md.Parameters[0].ParameterType.FullName == "System.String");

			// The old Forms.Color was a struct, so the following BuildException wasn't thrown because IsValueType returned true.
			// MGColor is a class, so IsValueType is false, but there's no parameterless constructor or factory; there's a 
			// TypeConverter. So adding this bool as a check for this condition temporarily.
			bool isColor = typedef.FullName == "Microsoft.Maui.Graphics.Color";

			if (!isColor && !typedef.IsValueType && ctorInfo == null && factoryMethodInfo == null)
			{
				throw new BuildException(BuildExceptionCode.ConstructorDefaultMissing, node, null, typedef.FullName);
			}

			if (isColor || ctorinforef != null || factorymethodinforef != null || typedef.IsValueType)
			{
				VariableDefinition vardef = new VariableDefinition(typeref);
				Context.Variables[node] = vardef;
				Context.Body.Variables.Add(vardef);

				ValueNode vnode = null;
				if (node.CollectionItems.Count == 1 && (vnode = node.CollectionItems.First() as ValueNode) != null &&
					(vardef.VariableType.IsValueType || isColor))
				{
					//<Color>Purple</Color>
					Context.IL.Append(vnode.PushConvertedValue(Context, typeref, [typedef],
						(requiredServices) => node.PushServiceProvider(Context, requiredServices),
						false, true));
					Context.IL.Emit(Stloc, vardef);
				}

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Add a public parameterless constructor to the type.
2. If you cannot change the type, instantiate it in code-behind or via a static x:FactoryMethod.
3. Decorate a parameterized ctor's params with [Parameter] and supply them in XAML.
4. If a TypeConverter should apply, ensure it has ProvideCompiledAttribute so XAML compilation uses it.
5. For value types the error is suppressed; consider whether the type can be a struct, though that is a design change.

Example fix

// before
public class MyService { public MyService(IConfiguration cfg) {...} }  <!-- no default ctor -->
<MyService />
// after
public class MyService {
  public MyService() {}            // added default ctor
  public MyService(IConfiguration cfg) {...}
}

// A type usable in XAML should have a public parameterless ctor
static bool IsXamlInstantiable(Type t) =>
    t.IsValueType || t == typeof(Microsoft.Maui.Graphics.Color)
    || t.GetConstructor(Type.EmptyTypes) != null;

When it happens

Trigger: Instantiating a class in XAML that only has non-public or parameterized constructors without [Parameter]; a third-party type not designed for XAML instantiation; a type that expects a TypeConverter but is not the Color special case.

Common situations: Trying to declare <ThirdPartyType /> in XAML where the type has no default ctor; using a model class meant for code-behind; missing the TypeConverter attribute the compiler would otherwise use.

Related errors

• XC0005
• XC0003
• XC0004
• XC0006
• XC0040

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/97d94d1bae586c98. Report an issue: GitHub.