dotnet/wpf · error · InvalidOperationException

SR.StyleForWrongType

Error message

SR.StyleForWrongType

What it means

GroupItem.PrepareItemContainer applies the ItemContainerStyle (or generator-provided style) to the GroupItem, but first verifies the style's TargetType is compatible with the GroupItem's actual type. If style.TargetType is not an instance-of type of the container, InvalidOperationException is thrown, since applying an incompatible style would cause property setter failures.

Solutions

  1. Set the Style's TargetType to GroupItem or a base type of the actual container
  2. Use x:Key'd styles applied explicitly to matching containers
  3. Remove TargetType constraints or use BasedOn correctly when deriving containers

Example fix

<!-- before -->
<Style TargetType="ContentPresenter"> ... </Style>
<!-- after -->
<Style TargetType="GroupItem"> ... </Style>
Defensive patterns

Strategy: validation

Validate before calling

if (style != null && style.TargetType != null && !style.TargetType.IsInstanceOfType(groupItem)) throw new InvalidOperationException("Style TargetType does not match container");

Type guard

bool StyleFits(Style s, object o) => s?.TargetType == null || s.TargetType.IsInstanceOfType(o);

Try / catch

try { groupItem.Style = style; } catch (InvalidOperationException ex) when (ex.Message.Contains("TargetType") || ex.Message.Contains("style")) { /* fix TargetType or use matching style */ }

Prevention

When it happens

Trigger: An ItemsControl's GroupStyle or ItemContainerStyle targets a type other than the actual container (e.g. a Style whose TargetType is GroupItem subclass A used where container is GroupItem subclass B, or a style targeting ContentPresenter/other type applied to GroupItem).

Common situations: Copy-pasting a Style between container types; changing the container type (e.g. custom GroupItem) without updating Style TargetType; implicit style resolution picking a style typed for a different class.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/346cd5b155fdefe9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/GroupItem.cs:183

            // apply the container style
            Style style = groupStyle.ContainerStyle;

            // no ContainerStyle set, try ContainerStyleSelector
            if (style == null)
            {
                if (groupStyle.ContainerStyleSelector != null)
                {
                    style = groupStyle.ContainerStyleSelector.SelectStyle(item, this);
                }
            }

            // apply the style, if found
            if (style != null)
            {
                // verify style is appropriate before applying it
                if (!style.TargetType.IsInstanceOfType(this))
                    throw new InvalidOperationException(SR.Format(SR.StyleForWrongType, style.TargetType.Name, this.GetType().Name));

                this.Style = style;
                this.WriteInternalFlag2(InternalFlags2.IsStyleSetFromGenerator, true);
            }

            // forward the header template information
            if (ContentIsItem || !HasNonDefaultValue(ContentProperty))
            {
                this.Content = item;
                ContentIsItem = true;
            }
            if (!HasNonDefaultValue(ContentTemplateProperty))
                this.ContentTemplate = groupStyle.HeaderTemplate;
            if (!HasNonDefaultValue(ContentTemplateSelectorProperty))
                this.ContentTemplateSelector = groupStyle.HeaderTemplateSelector;
            if (!HasNonDefaultValue(ContentStringFormatProperty))
                this.ContentStringFormat = groupStyle.HeaderStringFormat;

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