What causes "SR.CollectionCannotContainNulls" in dotnet/wpf?

Filter or assert all TypeArguments are non-null before calling GetXamlType

dotnet/wpf · error · ArgumentException

SR.CollectionCannotContainNulls

Error message

SR.CollectionCannotContainNulls

What it means

When resolving a generic XamlTypeName, each entry of xamlTypeName.TypeArguments must be non-null; a null type argument makes the generic type unresolvable, so ArgumentException with CollectionCannotContainNulls is thrown.

Solutions

Filter or assert all TypeArguments are non-null before calling GetXamlType
Resolve each type argument through GetXamlType first and handle null returns
Log which index was null to find the producer of the bad argument

Example fix

// before
tn.TypeArguments.Add(null);
// after
foreach (var arg in args) if (arg != null) tn.TypeArguments.Add(arg);

Defensive patterns

Strategy: validation

Validate before calling

if (xamlTypeName.TypeArguments?.Contains(null) == true) throw new ArgumentException("TypeArguments must not contain null entries");

Type guard

bool HasValidTypeArgs(XamlTypeName t) => t?.TypeArguments is null || t.TypeArguments.All(a => a is not null);

Try / catch

try { var xt = ctx.GetXamlType(xtn); } catch (ArgumentException ex) when (ex.Message.Contains("TypeArguments")) { /* handle null generic argument */ }

Prevention

Resolve each generic argument to a XamlType before composing
Assert generic arity matches supplied arguments

When it happens

Trigger: Calling GetXamlType on a XamlTypeName with TypeArguments populated including a null element (e.g. building the list by index and leaving a slot null).

Common situations: Programmatic construction of generic type names (List<T>, Dictionary<K,V> in XAML) where one argument failed to resolve to a XamlType beforehand.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b2477ca3212e8d5a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlSchemaContext.cs:295

            if (xamlTypeName.Name is null)
            {
                throw new ArgumentException(SR.Format(SR.ReferenceIsNull, "xamlTypeName.Name"), nameof(xamlTypeName));
            }

            if (xamlTypeName.Namespace is null)
            {
                throw new ArgumentException(SR.Format(SR.ReferenceIsNull, "xamlTypeName.Namespace"), nameof(xamlTypeName));
            }

            XamlType[] typeArgs = null;
            if (xamlTypeName.HasTypeArgs)
            {
                typeArgs = new XamlType[xamlTypeName.TypeArguments.Count];
                for (int i = 0; i < xamlTypeName.TypeArguments.Count; i++)
                {
                    if (xamlTypeName.TypeArguments[i] is null)
                    {
                        throw new ArgumentException(SR.Format(SR.CollectionCannotContainNulls, "xamlTypeName.TypeArguments"));
                    }

                    typeArgs[i] = GetXamlType(xamlTypeName.TypeArguments[i]);
                    if (typeArgs[i] is null)
                    {
                        return null;
                    }
                }
            }

            return GetXamlType(xamlTypeName.Namespace, xamlTypeName.Name, typeArgs);
        }

        protected internal virtual XamlType GetXamlType(string xamlNamespace, string name, params XamlType[] typeArguments)
        {
            ArgumentNullException.ThrowIfNull(xamlNamespace);
            ArgumentNullException.ThrowIfNull(name);
            if (typeArguments is not null)

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