dotnet/aspnetcore · error · InvalidOperationException

The property '{parameterName}' on component type '{targetTyp

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

Thrown by ComponentProperties.ThrowForCaptureUnmatchedValuesConflict when a component has a [Parameter(CaptureUnmatchedValues = true)] property that the user sets explicitly in markup AND there are also unmatched (splat) attributes destined for the same dictionary. The framework refuses to either mutate the user-supplied value or silently copy it.

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);

View on GitHub (pinned to 294cab2f9b)

Solutions

Do not set the CaptureUnmatchedValues parameter explicitly if you also splat other attributes - let the framework populate it.
If you need to merge, combine the dictionaries in code and pass only the merged dictionary, removing the extra unmatched attributes from markup.
Use @attributes="@attrs" on an element inside the component rather than binding the component's own catch-all.

Example fix

// before
<Child Attributes="@attrs" data-foo="bar" />

// after - merge in code, no extra splatted attribute
@code {
    private readonly Dictionary<string, object> _merged =
        new(attrs) { ["data-foo"] = "bar" };
}
<Child Attributes="@_merged" />

Defensive patterns

Strategy: validation

Validate before calling

// Do not set the CaptureUnmatchedValues parameter explicitly when also splatting attrs.
// Detect the catch-all parameter name to warn callers.
static string? GetCaptureUnmatchedValuesPropertyName(Type componentType)
{
    foreach (var p in componentType.GetProperties())
    {
        if (p.GetCustomAttribute<ParameterAttribute>()?.CaptureUnmatchedValues == true)
            return p.Name;
    }
    return null;
}

Type guard

static bool IsCaptureUnmatchedValuesProperty(PropertyInfo p)
    => p.GetCustomAttribute<ParameterAttribute>()?.CaptureUnmatchedValues == true;

Prevention

Never bind the catch-all parameter explicitly while also splatting other attributes.
Merge attribute dictionaries in code and pass a single dictionary.
Use @attributes on inner elements instead of binding the component's own catch-all.
Keep the catch-all parameter name documented so callers avoid direct binding.

When it happens

Trigger: <Child Attributes="@myDict" extra-attr="x" /> where Attributes is the CaptureUnmatchedValues parameter and extra-attr is unmatched.

Common situations: Combining an explicit attributes pass-through with additional splatted attributes on the same element; forwarding @attributes while also binding the catch-all parameter directly.

Related errors

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/1f65d4408ae1f384. Report an issue: GitHub.