unoplatform/uno · error · InvalidOperationException

Could not lookup prefix for namespace '{0}'

Error message

Could not lookup prefix for namespace '{0}'

What it means

Thrown by XamlTypeName.ToString(INamespacePrefixLookup) when a non-null prefixLookup is provided but LookupPrefix returns null for the XamlTypeName's namespace. The prefix-based serialization format requires a known prefix for every namespace; a missing mapping means the XAML prefix table is incomplete relative to the types being serialized.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeName.cs:251

		public override string ToString ()
		{
			return ToString (null);
		}

		public string ToString (INamespacePrefixLookup prefixLookup)
		{
			if (Namespace == null)
				throw new InvalidOperationException ("Namespace must be set before calling ToString method.");
			if (Name == null)
				throw new InvalidOperationException ("Name must be set before calling ToString method.");

			string ret;
			if (prefixLookup == null)
				ret = String.Concat ("{", Namespace, "}", Name);
			else {
				string p = prefixLookup.LookupPrefix (Namespace);
				if (p == null)
					throw new InvalidOperationException (String.Format (CultureInfo.InvariantCulture, "Could not lookup prefix for namespace '{0}'", Namespace));
				ret = p.Length == 0 ? Name : p + ":" + Name;
			}
			string arr = null;
			if (ret [ret.Length - 1] == ']') {
				int idx = ret.LastIndexOf ('[');
				arr = ret.Substring (idx);
				ret = ret.Substring (0, idx);
			}

			if (TypeArguments.Count > 0)
				ret += String.Concat ("(", DoToString (TypeArguments, prefixLookup), ")");

			return ret + arr;
		}
	}
}

View on GitHub (pinned to 0418340488)

Solutions

1. Ensure every namespace used by your XamlTypeName objects has a corresponding xmlns prefix registered in the INamespacePrefixLookup before serialization.
2. Call ToString() without arguments (pass null for prefixLookup) to get the brace-format form ({ns}Name) which does not require prefix resolution.
3. Check prefixLookup.LookupPrefix(ns) before calling ToString and register the missing namespace if it returns null.

Example fix

// before
string output = xamlTypeName.ToString(prefixLookup); // prefixLookup missing the namespace

// after
if (prefixLookup.LookupPrefix(xamlTypeName.Namespace) == null)
    // register or fall back to brace format
    output = xamlTypeName.ToString(); // {ns}Name
else
    output = xamlTypeName.ToString(prefixLookup);

if (prefixLookup.LookupPrefix(tn.Namespace) == null)
    return tn.ToString(); // brace format fallback
return tn.ToString(prefixLookup);

static bool HasPrefix(INamespacePrefixLookup lookup, string ns)
    => lookup?.LookupPrefix(ns) != null;

try { return tn.ToString(prefixLookup); }
catch (InvalidOperationException) { return tn.ToString(); } // fallback to {ns}Name

When it happens

Trigger: Calling ToString(prefixLookup) where prefixLookup is a PrefixLookup (or custom INamespacePrefixLookup) that was built from a different or smaller set of xmlns declarations than the XamlTypeName's Namespace expects.

Common situations: Serializing a type whose namespace was not declared in the source XAML document. Mixing XamlTypeName objects from one schema context with a prefix lookup from another. XAML namespace aliasing or remapping where the prefix table wasn't updated to match.

Related errors

• prefixLookup
• namespaceResolver
• typeArguments array contains one or more null XamlTypeName
• Namespace must be set before calling ToString method.
• Name must be set before calling ToString method.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/2c4b92b44e88cca0. Report an issue: GitHub.