dotnet/wpf · error · InvalidOperationException

SR.Format(SR.MissingContentSource, contentSource…

Error message

SR.Format(SR.MissingContentSource, contentSource, targetType)

What it means

TemplateContent.SetPropertyValMap-style code resolves properties named '{contentSource}', '{contentSource}Template', etc. on the target type via DependencyProperty.FromName. When the base content property (e.g. 'Content') does not exist on the target type even though a content source was specified, it throws InvalidOperationException (SR.MissingContentSource).

Solutions

Ensure the templated control actually defines the content property referenced by ContentSource (e.g. inherit from ContentControl or define 'Content' DP).
Correct or remove the ContentSource attribute in the template.
Verify the template's TargetType matches the control being templated.

Example fix

// before
<ControlTemplate TargetType="ItemsControl" ... ContentSource="Content">  <!-- no Content DP -->
// after
<ControlTemplate TargetType="ContentControl" ... ContentSource="Content">

Defensive patterns

Strategy: validation

Validate before calling

bool hasContent = typeof(ContentControl).IsAssignableFrom(targetType) ||
    targetType.GetProperties().Any(p => p.Name == contentSource && typeof(DependencyProperty).IsAssignableFrom(AssociatedDP(p)));

Type guard

static bool SupportsContentSource(Type t, string source) => DependencyProperty.FromName(source, t) != null;

Try / catch

try { element.Template = template; }
catch (InvalidOperationException ex) { log(ex.Message); applyFallbackTemplate(element); }

Prevention

Only set ContentSource on controls that define the matching DependencyProperty
Verify TargetType before reusing templates across controls
Spell ContentSource exactly as the DP name

When it happens

Trigger: Using the ContentSource attribute in a template whose TargetType does not define the named content property — DependencyProperty.FromName returns null for the base property while isContentSourceSet is true.

Common situations: Writing a ControlTemplate for a control that lacks the expected Content/ContentTemplate/ContentStringFormat family, or a typo in the ContentSource value, or targeting the wrong type with a template built for ContentControl.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c42a49b3149dfdfd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/TemplateContent.cs:1236

            bool isContentStringFormatPropertyDefined
            )
        {
            if (String.IsNullOrEmpty(contentSource) && !isContentSourceSet)
                contentSource = "Content";

            if (!String.IsNullOrEmpty(contentSource) && !isContentPropertyDefined)
            {
                Debug.Assert(templateChildName != null);

                DependencyProperty dpContent = DependencyProperty.FromName(contentSource, targetType);
                DependencyProperty dpContentTemplate = DependencyProperty.FromName($"{contentSource}Template", targetType);
                DependencyProperty dpContentTemplateSelector = DependencyProperty.FromName(
                    $"{contentSource}TemplateSelector", targetType);
                DependencyProperty dpContentStringFormat = DependencyProperty.FromName($"{contentSource}StringFormat", targetType);

                if (dpContent == null && isContentSourceSet)
                {
                    throw new InvalidOperationException(SR.Format(SR.MissingContentSource, contentSource, targetType));
                }

                if (dpContent != null)
                {
                    PropertyValue pv = new PropertyValue
                    {
                        ValueType = PropertyValueType.TemplateBinding,
                        ChildName = templateChildName,
                        ValueInternal = new TemplateBindingExtension(dpContent),
                        Property = ContentPresenter.ContentProperty
                    };

                    StyleHelper.UpdateTables(ref pv, ref childRecordFromChildIndex,
                        ref triggerSourceRecordFromChildIndex,
                        ref resourceDependents,
                        ref dataTriggerRecordFromBinding,
                        childIndexFromChildName,
                        ref hasInstanceValues);

View on GitHub (pinned to 81131a70a4)