dotnet/wpf · error · ArgumentException

SR.Format(SR.MarkupExtensionBadStatic, _member)

Error message

SR.Format(SR.MarkupExtensionBadStatic, _member)

What it means

StaticExtension (the XAML {x:Static} markup extension) resolves a 'Type.Member' string to a static field or property value via reflection. ProvideValue throws ArgumentException with MarkupExtensionBadStatic when the Member string contains no '.' separator, because it cannot be split into a type part and a member part. The library requires the format 'prefix:TypeName.MemberName' (prefix optional).

Solutions

Set Member to a fully qualified 'Type.Member' string, e.g. 'local:MyClass.MyConstant' or 'System:String.Empty'.
Validate the member string contains a '.' before calling ProvideValue.
In XAML, fix the {x:Static} usage to include the declaring type, e.g. {x:Static sys:String.Empty}.
If using an XML prefix, ensure the prefix is declared and the type part maps to the declaring type.

Example fix

// before
var ext = new StaticExtension("MyConstant");
// after
var ext = new StaticExtension("MyNamespace.MyClass.MyConstant");

Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrEmpty(member) || !member.Contains('.')) throw new ArgumentException($"Member '{member}' must be in 'Type.Member' format");

Type guard

static bool IsValidStaticMember(string? m) => !string.IsNullOrEmpty(m) && m.Contains('.');

Try / catch

try { value = ext.ProvideValue(serviceProvider); } catch (ArgumentException ex) { /* log ex.Message; surface XAML error */ }

Prevention

Always write {x:Static ns:Class.Member} with the declaring type.
Validate member strings contain a dot before constructing StaticExtension.
Prefer MemberType + typed code paths when the Type is already known.

When it happens

Trigger: Calling new StaticExtension(member).ProvideValue(serviceProvider) with a Member string that contains no dot, e.g. Member="Button" or Member="" — dotIndex = _member.IndexOf('.') is < 0.

Common situations: XAML files with {x:Static SomeValue} missing the type qualifier; code-built markup extensions where the member string was assembled from user input or config and the type portion was dropped; refactors that renamed types leaving a bare member name.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3001bc99ce980e8c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/StaticExtension.cs:68

            {
                throw new InvalidOperationException(SR.MarkupExtensionStaticMember);
            }

            Type type = MemberType;
            string fieldString;
            string typeNameForError = null;
            if (type is not null)
            {
                fieldString = _member;
                typeNameForError = type.FullName;
            }
            else
            {
                // Validate the _member
                int dotIndex = _member.IndexOf('.');
                if (dotIndex < 0)
                {
                    throw new ArgumentException(SR.Format(SR.MarkupExtensionBadStatic, _member));
                }

                // Pull out the type substring (this will include any XML prefix, e.g. "av:Button")
                string typeString = _member.Substring(0, dotIndex);
                if (string.IsNullOrEmpty(typeString))
                {
                    throw new ArgumentException(SR.Format(SR.MarkupExtensionBadStatic, _member));
                }

                // Get the IXamlTypeResolver from the service provider

                ArgumentNullException.ThrowIfNull(serviceProvider);

                if (serviceProvider.GetService(typeof(IXamlTypeResolver)) is not IXamlTypeResolver xamlTypeResolver)
                {
                    throw new ArgumentException(SR.Format(SR.MarkupExtensionNoContext, GetType().Name, nameof(IXamlTypeResolver)));
                }

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