unoplatform/uno · error · InvalidOperationException

Value must be non-null string.

Error message

Value must be non-null string.

What it means

InvalidOperationException thrown by NameReferenceConverter.ConvertFrom (System.Xaml, ported from Mono) when the input value is null or an empty string. The converter returns a name reference string, so it requires a non-empty string input; null/empty is a usage error rather than an argument-null case (context was already validated).

Source

Thrown at src/SourceGenerators/System.Xaml/System.Windows.Markup/NameReferenceConverter.cs:60

			{
				return false;
			}

			var p = context.GetService (typeof (IXamlNameProvider)) as IXamlNameProvider;
			return p != null && destinationType == typeof (string);
		}

		public override object ConvertFrom (ITypeDescriptorContext context, CultureInfo culture, object value)
		{
			if (context == null)
			{
				throw new ArgumentNullException ("context");
			}

			var s = value as string;
			if (String.IsNullOrEmpty (s))
			{
				throw new InvalidOperationException ("Value must be non-null string.");
			}

			return s;
		}

		public override object ConvertTo (ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
		{
			if (context == null)
			{
				return null;
			}

			var p = context.GetService (typeof (IXamlNameProvider)) as IXamlNameProvider;
			return p?.GetName (value);
		}
	}
}

View on GitHub (pinned to 0418340488)

Solutions

1. Validate the input is a non-empty string before invoking ConvertFrom.
2. Coerce null/empty to a sensible default name upstream of the converter.
3. In XAML, ensure the name reference attribute has a value.

Example fix

// before
var name = (string)converter.ConvertFrom(ctx, culture, maybeEmpty);

// after
if (string.IsNullOrEmpty(maybeEmpty)) throw new ArgumentException("Name reference required.");
var name = (string)converter.ConvertFrom(ctx, culture, maybeEmpty);

static string SafeConvert(NameReferenceConverter c, ITypeDescriptorContext ctx, string value) { if (string.IsNullOrEmpty(value)) throw new ArgumentException("Name reference required.", nameof(value)); return (string)c.ConvertFrom(ctx, CultureInfo.InvariantCulture, value); }

static bool IsNonEmptyName(object value) => value is string s && s.Length > 0;

try { return (string)converter.ConvertFrom(ctx, culture, value); } catch (InvalidOperationException ex) when (ex.Message.Contains("non-null string")) { return string.Empty; }

When it happens

Trigger: Passing null, string.Empty, or whitespace-only value to ConvertFrom once a valid context is present.

Common situations: A bound/config value that resolves to null or empty being fed to the converter, XAML where the reference is omitted, or default(string) passed programmatically.

Related errors

• context
• Type property must be set before calling ProvideValue method
• Item in the array must be an instance of '{0}'
• Name property is not set
• serviceProvider does not implement IXamlNameResolver

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/6a1bf7a4e9b71c1a. Report an issue: GitHub.