dotnet/maui · error · ArgumentException

EnumPicker: EnumType property must be enumeration type

Error message

EnumPicker: EnumType property must be enumeration type

What it means

This sample EnumPicker (a Picker subclass) exposes an EnumType bindable property. When EnumType is set, the propertyChanged callback checks whether the assigned Type is an enum via IsEnum. If it is not (e.g., a class, struct, or interface), it throws ArgumentException because Enum.GetValues would fail or produce nonsensical results for a non-enum type.

Source

Thrown at src/Controls/samples/Controls.Sample/Controls/EnumPicker.cs:22

namespace Maui.Controls.Sample.Controls
{
	class EnumPicker : Picker
	{
		public static readonly BindableProperty EnumTypeProperty =
			BindableProperty.Create(nameof(EnumType), typeof(Type), typeof(EnumPicker),
				propertyChanged: (bindable, oldValue, newValue) =>
				{
					EnumPicker picker = (EnumPicker)bindable;

					if (oldValue != null)
					{
						picker.ItemsSource = null;
					}
					if (newValue != null)
					{
						if (!((Type)newValue).GetTypeInfo().IsEnum)
							throw new ArgumentException("EnumPicker: EnumType property must be enumeration type");

						picker.ItemsSource = Enum.GetValues((Type)newValue);
					}
				});

		public Type EnumType
		{
			set => SetValue(EnumTypeProperty, value);
			get => (Type)GetValue(EnumTypeProperty);
		}
	}
}

View on GitHub (pinned to f377ff1c5e)

Solutions

Only assign enumeration types to EnumType: `enumPicker.EnumType = typeof(MyEnum);`
Add a runtime guard before assignment: `if (typeof(T).GetTypeInfo().IsEnum) enumPicker.EnumType = typeof(T);`
Add a generic constraint `where T : struct, Enum` if EnumType is set from a generic method.

Example fix

// before
enumPicker.EnumType = typeof(MySettingsClass); // not an enum

// after
enumPicker.EnumType = typeof(MySettingsEnum); // actual enum

Defensive patterns

Strategy: validation

Validate before calling

var candidateType = typeof(MyEnum);
if (candidateType.GetTypeInfo().IsEnum)
    enumPicker.EnumType = candidateType;
else
    throw new InvalidOperationException($"{candidateType.Name} is not an enum.");

Type guard

static bool IsEnumType(Type? type) => type?.GetTypeInfo().IsEnum ?? false;

Prevention

Only assign typeof() of actual enum types to EnumType.
Add a generic constraint `where T : struct, Enum` when setting EnumType from generics.
Validate the type with IsEnum before assignment in dynamic scenarios.

When it happens

Trigger: Binding or assigning EnumType to a Type that is not an enumeration — e.g., `enumPicker.EnumType = typeof(string)` or `enumPicker.EnumType = typeof(MyClass)`. Setting EnumType from a data source that resolves to a non-enum Type.

Common situations: Binding EnumType to a property whose type is not constrained at compile time. Passing typeof(T) where T is not an enum generic constraint. Mistakenly assigning the value type rather than the enum type. Dynamic type resolution from reflection producing a non-enum Type.

Related errors

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/4880f3438b054c46. Report an issue: GitHub.