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

Use the exact enum member name as defined in the target enum type (case-sensitive on most platforms, though TryParse is case-sensitive by default here).

dotnet/maui · error · BuildException

XC0040

XC0040

Error message

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

What it means

XC0040 Conversion thrown by the generic EnumTypeConverter<TEnum> when a XAML string cannot be parsed as the target enum type. The converter trims the value and calls Enum.TryParse<TEnum>. If the value is null/empty or does not match any defined enum member (by name), the build-time exception fires.

Source

Thrown at src/Controls/src/Build.Tasks/CompiledConverters/EnumTypeConverter.cs:22

using Microsoft.Maui.Controls.Xaml;
using Mono.Cecil.Cil;

namespace Microsoft.Maui.Controls.XamlC
{
	class EnumTypeConverter<TEnum> : ICompiledTypeConverter where TEnum : struct
	{
		public IEnumerable<Instruction> ConvertFromString(string value, ILContext context, BaseNode node)
		{
			if (!string.IsNullOrEmpty(value))
			{
				value = value.Trim();
				if (Enum.TryParse(value, out TEnum enumValue))
				{
					yield return Instruction.Create(OpCodes.Ldc_I4, (int)(object)enumValue);
					yield break;
				}
			}
			throw new BuildException(BuildExceptionCode.Conversion, node, null, value, typeof(TEnum));
		}
	}
}

View on GitHub (pinned to f377ff1c5e)

Solutions

Use the exact enum member name as defined in the target enum type (case-sensitive on most platforms, though TryParse is case-sensitive by default here).
Check the IntelliSense or the enum definition for valid member names.
Ensure the value is not empty, null, or containing only whitespace.

Example fix

<!-- before -->
<Label HorizontalTextAlignment="Middle" />

<!-- after -->
<Label HorizontalTextAlignment="Center" />

Defensive patterns

Strategy: validation

Validate before calling

// Validate an enum string for a known enum type before building
static bool IsValidEnumValue<TEnum>(string value) where TEnum : struct
{
    return !string.IsNullOrWhiteSpace(value)
        && Enum.TryParse<TEnum>(value.Trim(), out _);
}

Type guard

static bool IsEnumValue<TEnum>(string value, out TEnum result) where TEnum : struct
    => Enum.TryParse(value?.Trim(), out result);

Prevention

Cross-check enum member names against the type definition or IntelliSense before writing them in XAML.
Enum.TryParse is case-sensitive in this converter — match the exact casing from the enum definition.
Avoid using integer values for enums in XAML; prefer named members for readability and resilience.

When it happens

Trigger: Setting an enum-backed property to a string that is not a member of the enum. For example, setting a TextAlignment to 'Justify' when the enum only defines Start, Center, End. Using integer values like '2' can work via TryParse but non-numeric, non-member strings always fail.

Common situations: Misspelling an enum member (e.g. 'Centr' instead of 'Center'). Using values from a different/older API version where the enum member was renamed or removed. Copying values from documentation for a different platform or framework.

Related errors

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/c302c38125e58ef0. Report an issue: GitHub.