dotnet/maui · error · BuildException

XC0043

XC0043

Error message

Binding: Unsupported indexer index type: "{0}".

What it means

Thrown when a compiled-binding indexer is resolved but its parameter type is neither string, Int32, nor an enum. The compiler only knows how to emit index lookups for those key types, so any other key type (Guid, double, custom struct, etc.) is rejected.

Source

Thrown at src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs:904

																	&& pd.GetMethod != null
																	&& TypeRefComparer.Default.Equals(pd.GetMethod.Parameters[0].ParameterType.ResolveGenericParameters(previousPartTypeRef), module.ImportReference(context.Cache, ("mscorlib", "System", "Object")))
																	&& pd.GetMethod.IsPublic, out indexerDeclTypeRef);
					// Try to find an indexer with an enum parameter type
					indexer ??= previousPartTypeRef.GetProperty(context.Cache,
																	pd => pd.Name == indexerName
																	&& pd.GetMethod != null
																	&& pd.GetMethod.Parameters[0].ParameterType.ResolveGenericParameters(previousPartTypeRef).ResolveCached(context.Cache)?.IsEnum == true
																	&& pd.GetMethod.IsPublic, out indexerDeclTypeRef);

					properties.Add((indexer, indexerDeclTypeRef, indexArg));
					if (indexer != null) //the case when we index on an array, not a list
					{
						var indexType = indexer.GetMethod.Parameters[0].ParameterType.ResolveGenericParameters(indexerDeclTypeRef);
						var indexTypeDef = indexType.ResolveCached(context.Cache);
						if (!TypeRefComparer.Default.Equals(indexType, module.TypeSystem.String) 
							&& !TypeRefComparer.Default.Equals(indexType, module.TypeSystem.Int32)
							&& indexTypeDef?.IsEnum != true)
							throw new BuildException(BindingIndexerTypeUnsupported, lineInfo, null, indexType.FullName);
						previousPartTypeRef = indexer.PropertyType.ResolveGenericParameters(indexerDeclTypeRef);
					}
					else
					{
						if (previousPartTypeRef.IsArray)
							previousPartTypeRef = previousPartTypeRef.GetElementType();

						previousPartTypeRef.ResolveCached(context.Cache);
					}

				}
			}
			pathProperties = properties;
			return true;
		}

		static IEnumerable<Instruction> DigProperties(IEnumerable<(PropertyDefinition property, TypeReference propDeclTypeRef, string indexArg)> properties, Dictionary<TypeReference, VariableDefinition> locs, Func<Instruction> fallback, IXmlLineInfo lineInfo, ModuleDefinition module, XamlCache cache = null)
		{

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Expose or bind to a string-, int-, or enum-keyed indexer instead.
2. Reshape the source so the data is reachable via a property or a supported key.
3. Fall back to a non-compiled (reflection) binding by removing x:DataType from that element.

Example fix

<!-- before (Guid-keyed indexer, unsupported) -->
<Label Text="{Binding Items[item.Guid], x:DataType=vm:Vm}" />

<!-- after: use a string key -->
<Label Text="{Binding Items[item.Key], x:DataType=vm:Vm}" />

// Compiled bindings support only string/int/enum indexer keys
static bool IsSupportedIndexKey(Type keyType)
    => keyType == typeof(string) || keyType == typeof(int) || keyType.IsEnum;

static bool IsCompiledBindingIndexable(Type collectionType, out Type keyType)
{
    keyType = collectionType.GetDefaultMembers()?.SelectMany(
        t => collectionType.GetProperties().Where(p => p.GetIndexParameters().Length == 1))
        .Select(p => p.GetIndexParameters()[0].ParameterType).FirstOrDefault();
    return keyType != null && IsSupportedIndexKey(keyType);
}

When it happens

Trigger: Binding to an indexer whose key type is unsupported, e.g. `{Binding Map[someGuid]}` where the indexer is keyed by Guid.

Common situations: Using a dictionary keyed by a non-primitive/custom type in a compiled binding; exposing an indexer with an unusual parameter type.

Related errors

• XC0041
• XC0042
• XC0044
• XC0102
• XC0020

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/5a6c3beab3d1e1d3. Report an issue: GitHub.