dotnet/maui · error · BuildException

XC0003

XC0003

Error message

Missing mandatory property "{0}" on "{1}".

What it means

Thrown when a type exposes a parameterized constructor whose parameters are all decorated with [Parameter] (the Maui parameterized-ctor convention), ValidateCtorArguments flagged a missing mandatory parameter, and no parameterless fallback constructor exists. The message names the missing property and the type.

Source

Thrown at src/Controls/src/Build.Tasks/CreateObjectVisitor.cs:168

																			 md.HasParameters &&
																			 md.Parameters.All(
																				 pd =>
																					 pd.CustomAttributes.Any(
																						 ca =>
																							 ca.AttributeType.FullName ==
																							 "Microsoft.Maui.Controls.ParameterAttribute")));
			}
			string missingCtorParameter = null;
			if (parameterizedCtorInfo != null && ValidateCtorArguments(parameterizedCtorInfo, node, out missingCtorParameter))
			{
				ctorInfo = parameterizedCtorInfo;
				//				IL_0000:  ldstr "foo"
				Context.IL.Append(PushCtorArguments(parameterizedCtorInfo.ResolveGenericParameters(typeref, Module), node));
			}
			ctorInfo ??= typedef.Methods.FirstOrDefault(md => md.IsConstructor && !md.HasParameters && !md.IsStatic);
			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);
			}

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Add the missing property/attribute named in the error message to the element.
2. Check the type's [Parameter]-decorated constructor to see which properties are mandatory.
3. If the type is yours, add a parameterless constructor or make the parameter optional.
4. Fix any typos in property names so they bind to the [Parameter] arguments.

Example fix

// before
<MyControl />
  <!-- MyControl(..) requires Text via [Parameter] -->
// after
<MyControl Text="Hello" />

// List the [Parameter]-mandatory properties of a type's parameterized ctor
static IEnumerable<string> MandatoryCtorParams(Type t) =>
    t.GetConstructors()
     .SelectMany(c => c.GetParameters()
         .Where(p => p.GetCustomAttribute<ParameterAttribute>() != null)
         .Select(p => p.Name!));

When it happens

Trigger: Declaring an element in XAML but omitting a required [Parameter]-marked ctor argument; typo in the property name so it isn't recognized; the type relies solely on a parameterized ctor and the XAML didn't supply all params.

Common situations: Using Maui controls/types that require constructor arguments (e.g. certain gestures or custom controls); partial XAML where a required attribute was dropped during refactor.

Related errors

• XC0005
• XC0004
• XC0004
• XC0006
• XC0040

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/8495055001dc446d. Report an issue: GitHub.