dotnet/aspnetcore · error · InvalidOperationException

The property ' ' on component type ' ' cannot be set…

Error message

The property '{parameterName}' on component type '{targetType.FullName}' cannot be set explicitly when also used to capture unmatched values. Unmatched values:

What it means

ThrowForCaptureUnmatchedValuesConflict fires when a single property is annotated with [Parameter(CaptureUnmatchedValues = true)] (making it the unmatched-attributes sink) AND the framework is also asked to set that same property explicitly with a direct value. The sink property cannot simultaneously hold explicit assigned state and unmatched attributes; Blazor rejects the conflicting usage to prevent ambiguous data.

Solutions

Do not pass the capture-sink property name explicitly in markup; let it receive only unmatched attributes.
If you need to set that dictionary explicitly, remove CaptureUnmatchedValues = true and handle unmatched attributes separately.
Rename the sink property to avoid collision with any explicitly-supplied parameter name.

Example fix

// before
// Component: [Parameter(CaptureUnmatchedValues = true)]
//            public Dictionary<string,object>? Attributes { get; set; }
<MyComponent Attributes="@(new Dictionary<string,object>())" />

// after
<MyComponent class="btn" data-foo="bar" /> // unmatched only; Attributes auto-populated

Defensive patterns

Strategy: validation

Validate before calling

// Detect capture-sink collision before rendering.
var sinkProp = typeof(MyComponent).GetProperties()
    .FirstOrDefault(p => p.GetCustomAttribute<ParameterAttribute>()?.CaptureUnmatchedValues == true);
if (sinkProp != null && suppliedParameterNames.Contains(sinkProp.Name))
    throw new InvalidOperationException($"Do not pass '{sinkProp.Name}' explicitly; it captures unmatched values.");

Prevention

Never pass the capture-sink property name explicitly in markup.
Name the sink property to avoid collision (e.g., 'Attributes', 'AdditionalAttributes').
Document the sink's role so consumers splat attributes rather than setting it.

When it happens

Trigger: Declaring [Parameter(CaptureUnmatchedValues = true)] public Dictionary<string, object> Attributes { get; set; } and then in markup writing <MyComponent Attributes="..." /> (passing Attributes explicitly). The framework detects both an explicit assignment target and the unmatched-capture role on the same property.

Common situations: Adding a splat-attributes sink named 'Attributes' and then accidentally also setting 'Attributes' as a regular parameter in markup. Component libraries whose public surface exposes the capture property by a name consumers also pass.

Related errors

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/1f65d4408ae1f384. Report an issue: GitHub.

Appendix: source

Thrown at src/Components/Components/src/Reflection/ComponentProperties.cs:240

    private static void ThrowForSettingCascadingParameterWithNonCascadingValue(Type targetType, string parameterName)
    {
        throw new InvalidOperationException(
            $"The property '{parameterName}' on component type '{targetType.FullName}' cannot be set " +
            $"explicitly because it only accepts cascading values.");
    }

    [DoesNotReturn]
    private static void ThrowForSettingParameterWithCascadingValue(Type targetType, string parameterName)
    {
        throw new InvalidOperationException(
            $"The property '{parameterName}' on component type '{targetType.FullName}' cannot be set " +
            $"using a cascading value.");
    }

    [DoesNotReturn]
    private static void ThrowForCaptureUnmatchedValuesConflict(Type targetType, string parameterName, Dictionary<string, object> unmatched)
    {
        throw new InvalidOperationException(
            $"The property '{parameterName}' on component type '{targetType.FullName}' cannot be set explicitly " +
            $"when also used to capture unmatched values. Unmatched values:" + Environment.NewLine +
            string.Join(Environment.NewLine, unmatched.Keys));
    }

    [DoesNotReturn]
    private static void ThrowForMultipleCaptureUnmatchedValuesParameters([DynamicallyAccessedMembers(Component)] Type targetType)
    {
        var propertyNames = new List<string>();
        foreach (var property in targetType.GetProperties(BindablePropertyFlags))
        {
            if (property.GetCustomAttribute<ParameterAttribute>()?.CaptureUnmatchedValues == true)
            {
                propertyNames.Add(property.Name);
            }
        }

        propertyNames.Sort(StringComparer.Ordinal);

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