How can I supply a func to a method, so I could write something like:
MethodTest(a => a.IsAltTagAvailable);
Where the signature of this method takes a func which returns an object (say HtmlImage) when the condition is met (basically just a predicate).
Edit: I need to pass the type I will be working on as T (Parameter). I forgot to do this, how clumsy!
Thanks
A predicate tends to return bool, not an object. What are you going to return when the condition isn't met? Given your example, you don't really mean the function returns an object - you mean it takes an object and returns a bool.
Note that if you're going to have a parameter in the lambda expression, you'll need to use a delegate which takes parameters too.
We really need more information before giving a definitive answer, but you might want something like:
void MethodTest(Func<HtmlImage, bool> predicate)
or
void MethodTest(Predicate<HtmlImage> predicate)
(Personally I like the descriptive nature of using a named delegate, but others prefer to use Func/Action for almost everything.)
That's assuming that the type of input is fixed. If not, you might want to make it a generic method:
void MethodTest<T>(Predicate<T> predicate)
void MethodTest(Func<HtmlImage> func) {
}
void MethodTest(Func<HtmlImage, object> func)
{
}
HtmlImage is the argument of the function (x), object the return value, you could take the concrete type if you want to specify it.
void MethodTest(Func<HtmlImage, bool> func)
Which is a predicate:
void MethodTest(Predicate<HtmlImage> func)
To make it fully generic, replace HtmlImage with a generic argument:
void MethodTest<T>(Predicate<T> func)
public void MethodTest(Func<HtmlImage> delegate)
{
//do what you want
}
OR:
public delegate HtmlImageTagHandler(HtmlImage image);
public HtmlImage MethodTest(HtmlImageTagHandler handler, HtmlImage image)
{
return handler(image) == true ? image : null;
}
use:
MethodTest(a => a.IsAltTagAvailable, a);
Related
Occasionally I run into the problem of needing to execute a generic method without knowing its type.
I know I can do this each time using reflection, however I am trying to write a helper method:
public static object InvokeGeneric<T>(this T #this,
Expression<Func<T, object>> method,
Type genericType,
params object[] arguments)
{
// I think I know what to do here
// look at the expression tree, grab
// the method info, do the
// reflection in here, etc.
return null;
}
So that I can do this:
this._myService.InvokeGeneric(
e => e.MyGenericMethod, // interface IMyService { void MyGenericMethod<T>(T t); }
typeof(MyGenericType),
myArg);
However I am getting this error: Cannot convert method group 'XXX' to non-delegate type 'object'.
Without changing my calling syntax, how can I change my method signature of my helper method to do what I want?
EDIT:
I got it down to:
this._myService.InvokeGeneric<IMyService, object, MyArgType>(e => e.MyGenericMethod, typeof(MyGenericType), myArg);
The downside (besides extra typing) is that you'd need an overload for each generic variation of Func<> and Action<> that you'd want to support.
public static object InvokeGeneric<T, T1>(this object #this, Expression<Func<T, Action<T1>>> method, Type genericType, params object[] arguments)
{ }
public static object InvokeGeneric<T, T1, T2>(this object #this, Expression<Func<T, Action<T1, T2>>> method, Type genericType, params object[] arguments)
{ }
etc. I'm going to use that solution, but if anyone has something that meets the briefer syntax let me know and I'll accept it. Reading a bit about method groups made me realize my syntax is ambiguous, potentially, if there are overloads, meaning a strongly-typed one like this is probably better, anyway.
I'm trying to create an extension method for Moq where I can send in an expression to be used in an async return function. However this question is not really Moq specific. Here's what I have so far:
public static IReturnsResult<TMock> ReturnsAsync<TMock, TResult, T>(this IReturns<TMock, Task<TResult>> setup, Func<T, TResult> valueFunc) where TMock : class
{
return setup.Returns(Task.FromResult(valueFunc.Invoke(default(T))));
}
This is how I'm hoping to use it.
repo.Setup(x => x.FindAsync(It.IsAny<Expression<Func<T, bool>>>())).ReturnsAsync((Expression<Func<T, bool>> e) => context.GetSet<T>().FirstOrDefault(e));
Now I don't really know how all of this works and the thing I can't figure out is how to I get the expression passed on into the ReturnsAsync function so I can use it as the argument instead of the default(T) that I put there as a placeholder.
As expected the "e" variable here becomes null.
This method will do what you want:
public static IReturnsResult<TMock> ReturnsAsync<TMock, TResult, T>(
this IReturns<TMock, Task<TResult>> setup,
Func<Expression<Func<TResult, T>>, TResult> valueFunc)
where TMock : class
{
return setup.Returns<Expression<Func<TResult, T>>>(
e => Task.FromResult(valueFunc(e)));
}
Then use it like so:
repo.Setup(x => x.FindAsync(It.IsAny<Expression<Func<T, bool>>>()))
.ReturnsAsync<IRepository, int, bool>(e => context.GetSet<T>().FirstOrDefault(e));
Essentially, this version of ReturnsAsync takes a function that expects a predicate function (which is e) and returns a T. This allows you to then execute the predicate against your test data set (context.GetSet<T>.FirstOrDefault). Also, I used the overload of Returns that accepts a type parameter; this is used to forward the arguments from the Setup call to the function specified as the Returns argument.
Your version's signature only specified the predicate, so you had no way to execute it against your test data. You also had the T and TResult type parameters backwards in the valueFunc parameter's type.
There is a generic function with a return type.
TResult Invoke<TResult>(Func<string, TResult> callback)
{
string message = Generate_some_string();
return callback(message);
}
And also there is a similar one without a return type since there is no Func.
void Invoke(Action<string> callback)
{
string message = Generate_some_string();
callback(message);
}
But these are duplicate code. Once Invoke changes, Invoke has to be changed correspondingly. Is there any way to eliminate the duplicate code?
Thanks,
Jim
You could try something like this:
void Invoke(Action<string> callback)
{
Invoke<int>(s=>{callback(s);return 0;});
}
That way all your logic stays in the Func version and your Action version should never have to change.
Another option would be to create a ToFunc conversion routine, and place the onus on your callers to change their action into a func:
public static Func<TIn, TResult> ToFunc<TIn, TResult>(this Action<TIn> a)
{
return input =>
{
a(input);
return default(TResult);
};
}
I'm trying to pass an expression that describes a method but I want the argument to be strongly typed and I don't want to have to know the method signature or pass the arguments in the expression, something like this:
GetMethod<MyClass>(c => c.DoSomething);
Where DoSomething could have a method signature like this... string DoSomething(int id, int count)
I know I can do something like this:
MemberInfo GetMethod<T>(Expression<Func<T, Delegate>> expression);
//implementation
GetMethod<MyClass>(c => new Func<int, int, string>(c.DoSomething))
But frankly, this is quite ugly.
Is this possible?
Just have an overload for each possible Action/Func. It won't cover all possibilities (have an extra overload that you've shown there to cover all edge cases) but it'll handle most of them.
The body of each of the action/func overloads can just call the overload that you've shown above for the actual implementation.
public MemberInfo GetMethod<T1, T2>(Expression<Func<T1, Func<T2>>> expression)
{
return GetMethodImpl(expression);
}
public MemberInfo GetMethod<T1, T2, T3>(Expression<Func<T1, Func<T2, T3>>> expression)
{
return GetMethodImpl(expression);
}
public MemberInfo GetMethod<T1, T2>(Expression<Func<T1, Action<T2>>> expression)
{
return GetMethodImpl(expression);
}
//...
GetMethodImpl can then be implemented like so:
private MemberInfo GetMethodImpl<T1, T2>(Expression<Func<T1, T2>> expression)
{
}
That will be able to be just a slight modification of your existing GetMethod implementation. T2 will be your delegate; you may need to cast it to Delegate, depending on how you use it.
Func<> is very convenient in .NET. Is there a way i can specify the param type and have the result value as void? I'd like to pass void Write(string) as a parameter.
Action<T> - "Encapsulates a method that takes a single parameter and does not return a value"
I believe you're looking for the Action<T> family of delegate types.
It's not perfect, but sometimes when I want to fake this behavior against an existing function (and I'd rather not re-implement it as Action<TResult>) I'll just return null and throw the value away.
Func<T, TResult> myFunc = (inVar) =>
{
// do work...
return null as object;
};