dotnet/aspnetcore · error · InvalidOperationException

The event handler parameter type {declaredType.FullName} for

Error message

The event handler parameter type {declaredType.FullName} for event must inherit from {typeof(EventArgs).FullName}.

What it means

Blazor event handlers that accept a single parameter require that parameter to be assignable to System.EventArgs (or one of its subclasses like MouseEventArgs, KeyboardEventArgs, ChangeEventArgs). When EventArgsTypeCache.GetEventArgsType encounters a single-parameter method whose parameter type does not inherit from EventArgs, it throws InvalidOperationException. This ensures type safety for DOM event dispatch.

Source

Thrown at src/Components/Components/src/RenderTree/EventArgsTypeCache.cs:44

            var parameterInfos = methodInfo.GetParameters();
            if (parameterInfos.Length == 0)
            {
                return typeof(EventArgs);
            }
            else if (parameterInfos.Length > 1)
            {
                throw new InvalidOperationException($"The method {methodInfo} cannot be used as an event handler because it declares more than one parameter.");
            }
            else
            {
                var declaredType = parameterInfos[0].ParameterType;
                if (typeof(EventArgs).IsAssignableFrom(declaredType))
                {
                    return declaredType;
                }
                else
                {
                    throw new InvalidOperationException($"The event handler parameter type {declaredType.FullName} for event must inherit from {typeof(EventArgs).FullName}.");
                }
            }
        });
    }
}

View on GitHub (pinned to 294cab2f9b)

Solutions

Use the correct EventArgs subclass for the event type: MouseEventArgs for @onclick/@onmouseover, KeyboardEventArgs for @onkeydown/@onkeyup, ChangeEventArgs for @onchange/@oninput.
If you need to pass custom data, use a lambda closure instead: @onclick="@(() => Handler(myData))".
If no event argument is needed, declare the handler with zero parameters.
Check the Blazor docs for the correct EventArgs subclass mapping for each DOM event.

Example fix

<!-- before -->
@onclick="HandleClick"
@code {
    void HandleClick(int id) { } // int is not EventArgs
}

<!-- after -->
@onclick="@(e => HandleClick(itemId))"
@code {
    void HandleClick(int id) { }
}

Defensive patterns

Strategy: validation

Validate before calling

// Validate that a handler's single parameter inherits from EventArgs
static bool HasValidEventArgsParameter(MethodInfo method)
{
    var parms = method.GetParameters();
    if (parms.Length != 1) return parms.Length == 0;
    return typeof(EventArgs).IsAssignableFrom(parms[0].ParameterType);
}

Type guard

// In Razor, use the correct EventArgs subclass implicitly:
// @onclick="Handler" where void Handler(MouseEventArgs e) { }

Prevention

Use the correct EventArgs subclass for each DOM event (MouseEventArgs for click, KeyboardEventArgs for key events, etc.).
For custom data, use lambda closures instead of changing the parameter type.
Keep a reference of Blazor event-to-EventArgs mappings handy.
Run a Roslyn analyzer to validate handler parameter types at build time.

When it happens

Trigger: Using a method as a Blazor event handler (@onclick="MyMethod") where MyMethod accepts a single parameter whose type is not an EventArgs subclass, e.g., void MyMethod(int id) or void MyMethod(MyCustomDto data). The framework resolves the type via reflection and rejects non-EventArgs types.

Common situations: Confusion about Blazor event handler signatures; trying to pass custom data objects as the event argument; migrating from a different framework where arbitrary types were allowed; using a method that takes a primitive type (int, string) as the event handler.

Related errors

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/1fae009e01f1e27c. Report an issue: GitHub.