Blazor Cascading Parameters don’t “just work” with lambdas or method callback

TL;DR: Blazor Cascading Parameters are matched by Type not necessarily by Name. The simplest solution is to declare the Parameter type as Delegate. If you need to distinguish more than one delegate cascading parameter, then declare a named delegate type (that's a one-liner in C#) and use that as the parameter type.

The problem

You want to pass a callback as a cascading parameter. You try passing a lambda, or possibly a method, and create a CascadingParameter property, with matching name, of type Action or Func<...> or similar, to receive it. But the cascading parameter is always set to null.

Cascading parameters are matched up by Type, not by name, and who knows what the compiled type of a lambda will be? The C# reference tells you that that lambdas can be converted to delegates or expression trees but 'can be converted to' isn't good enough for matching up by Type.

The quickest fix

Declare your parameter to be of type Delegate. Most callable things in C# are of type Delegate.

// Declaration in the child component
[CascadingParameter]public Delegate OnChange { get; set; }

// Use in the child component. The null is what DynamicInvoke requires for 'no parameters'
OnChange.DynamicInvoke(null);

# In the parent container
<CascadingValue Value="StateHasChanged" >

If that's not good enough

If you must distinguish between several Delegate parameters, or just want to better express intent in your code, you can define a named delegate:

public delegate void StateHasChangedHook();

and use that as the Type of your CascadingParameter:

// Declaration in the child component
[CascadingParameter]public StateHasChangedHook OnChange { get; set; }

// Use in the child component is slightly simpler
OnChange();

# In the parent container
<CascadingValue Value="(StateHasChangedHook)this.StateHasChanged" >

When you use named delegates, you can invoke them with onChange() syntax instead of .Invoke() or DynamicInvoke(null).

You can inspect what was set in the child parameter

log.LogDebug("OnChange was set {OnChange}.MethodInfo={MethodInfo}",OnChange,OnChange?.GetMethodInfo())

Recall that Action<> and Func<> are themselves named delegates, not types. They are declared in the System assembly as for instance: public delegate void Action<in T>(T obj)