General purpose FromEvent method

Here you go:

internal class TaskCompletionSourceHolder
{
    private readonly TaskCompletionSource<object[]> m_tcs;

    internal object Target { get; set; }
    internal EventInfo EventInfo { get; set; }
    internal Delegate Delegate { get; set; }

    internal TaskCompletionSourceHolder(TaskCompletionSource<object[]> tsc)
    {
        m_tcs = tsc;
    }

    private void SetResult(params object[] args)
    {
        // this method will be called from emitted IL
        // so we can set result here, unsubscribe from the event
        // or do whatever we want.

        // object[] args will contain arguments
        // passed to the event handler
        m_tcs.SetResult(args);
        EventInfo.RemoveEventHandler(Target, Delegate);
    }
}

public static class ExtensionMethods
{
    private static Dictionary<Type, DynamicMethod> s_emittedHandlers =
        new Dictionary<Type, DynamicMethod>();

    private static void GetDelegateParameterAndReturnTypes(Type delegateType,
        out List<Type> parameterTypes, out Type returnType)
    {
        if (delegateType.BaseType != typeof(MulticastDelegate))
            throw new ArgumentException("delegateType is not a delegate");

        MethodInfo invoke = delegateType.GetMethod("Invoke");
        if (invoke == null)
            throw new ArgumentException("delegateType is not a delegate.");

        ParameterInfo[] parameters = invoke.GetParameters();
        parameterTypes = new List<Type>(parameters.Length);
        for (int i = 0; i < parameters.Length; i++)
            parameterTypes.Add(parameters[i].ParameterType);

        returnType = invoke.ReturnType;
    }

    public static Task<object[]> FromEvent<T>(this T obj, string eventName)
    {
        var tcs = new TaskCompletionSource<object[]>();
        var tcsh = new TaskCompletionSourceHolder(tcs);

        EventInfo eventInfo = obj.GetType().GetEvent(eventName);
        Type eventDelegateType = eventInfo.EventHandlerType;

        DynamicMethod handler;
        if (!s_emittedHandlers.TryGetValue(eventDelegateType, out handler))
        {
            Type returnType;
            List<Type> parameterTypes;
            GetDelegateParameterAndReturnTypes(eventDelegateType,
                out parameterTypes, out returnType);

            if (returnType != typeof(void))
                throw new NotSupportedException();

            Type tcshType = tcsh.GetType();
            MethodInfo setResultMethodInfo = tcshType.GetMethod(
                "SetResult", BindingFlags.NonPublic | BindingFlags.Instance);

            // I'm going to create an instance-like method
            // so, first argument must an instance itself
            // i.e. TaskCompletionSourceHolder *this*
            parameterTypes.Insert(0, tcshType);
            Type[] parameterTypesAr = parameterTypes.ToArray();

            handler = new DynamicMethod("unnamed",
                returnType, parameterTypesAr, tcshType);

            ILGenerator ilgen = handler.GetILGenerator();

            // declare local variable of type object[]
            LocalBuilder arr = ilgen.DeclareLocal(typeof(object[]));
            // push array's size onto the stack 
            ilgen.Emit(OpCodes.Ldc_I4, parameterTypesAr.Length - 1);
            // create an object array of the given size
            ilgen.Emit(OpCodes.Newarr, typeof(object));
            // and store it in the local variable
            ilgen.Emit(OpCodes.Stloc, arr);

            // iterate thru all arguments except the zero one (i.e. *this*)
            // and store them to the array
            for (int i = 1; i < parameterTypesAr.Length; i++)
            {
                // push the array onto the stack
                ilgen.Emit(OpCodes.Ldloc, arr);
                // push the argument's index onto the stack
                ilgen.Emit(OpCodes.Ldc_I4, i - 1);
                // push the argument onto the stack
                ilgen.Emit(OpCodes.Ldarg, i);

                // check if it is of a value type
                // and perform boxing if necessary
                if (parameterTypesAr[i].IsValueType)
                    ilgen.Emit(OpCodes.Box, parameterTypesAr[i]);

                // store the value to the argument's array
                ilgen.Emit(OpCodes.Stelem, typeof(object));
            }

            // load zero-argument (i.e. *this*) onto the stack
            ilgen.Emit(OpCodes.Ldarg_0);
            // load the array onto the stack
            ilgen.Emit(OpCodes.Ldloc, arr);
            // call this.SetResult(arr);
            ilgen.Emit(OpCodes.Call, setResultMethodInfo);
            // and return
            ilgen.Emit(OpCodes.Ret);

            s_emittedHandlers.Add(eventDelegateType, handler);
        }

        Delegate dEmitted = handler.CreateDelegate(eventDelegateType, tcsh);
        tcsh.Target = obj;
        tcsh.EventInfo = eventInfo;
        tcsh.Delegate = dEmitted;

        eventInfo.AddEventHandler(obj, dEmitted);
        return tcs.Task;
    }
}

This code will work for almost all events that return void (regardless of the parameter list).

It can be improved to support any return values if necessary.

You can see the difference between Dax's and mine methods below:

static async void Run() {
    object[] result = await new MyClass().FromEvent("Fired");
    Console.WriteLine(string.Join(", ", result.Select(arg =>
        arg.ToString()).ToArray())); // 123, abcd
}

public class MyClass {
    public delegate void TwoThings(int x, string y);

    public MyClass() {
        new Thread(() => {
                Thread.Sleep(1000);
                Fired(123, "abcd");
            }).Start();
    }

    public event TwoThings Fired;
}

Briefly, my code supports really any kind of delegate type. You shouldn't (and don't need to) specify it explicitly like TaskFromEvent<int, string>.

This will give you what you need without needing to do any ilgen, and way simpler. It works with any kind of event delegates; you just have to create a different handler for each number of parameters in your event delegate. Below are the handlers you'd need for 0..2, which should be the vast majority of your use cases. Extending to 3 and above is a simple copy and paste from the 2-parameter method.

This is also more powerful than the ilgen method because you can use any values created by the event in your async pattern.

// Empty events (Action style)
static Task TaskFromEvent(object target, string eventName) {
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var tcs = new TaskCompletionSource<object>();
    var resultSetter = (Action)(() => tcs.SetResult(null));
    var d = Delegate.CreateDelegate(delegateType, resultSetter, "Invoke");
    addMethod.Invoke(target, new object[] { d });
    return tcs.Task;
}

// One-value events (Action<T> style)
static Task<T> TaskFromEvent<T>(object target, string eventName) {
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var tcs = new TaskCompletionSource<T>();
    var resultSetter = (Action<T>)tcs.SetResult;
    var d = Delegate.CreateDelegate(delegateType, resultSetter, "Invoke");
    addMethod.Invoke(target, new object[] { d });
    return tcs.Task;
}

// Two-value events (Action<T1, T2> or EventHandler style)
static Task<Tuple<T1, T2>> TaskFromEvent<T1, T2>(object target, string eventName) {
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var tcs = new TaskCompletionSource<Tuple<T1, T2>>();
    var resultSetter = (Action<T1, T2>)((t1, t2) => tcs.SetResult(Tuple.Create(t1, t2)));
    var d = Delegate.CreateDelegate(delegateType, resultSetter, "Invoke");
    addMethod.Invoke(target, new object[] { d });
    return tcs.Task;
}

Use would be like this. As you can see, even though the event is defined in a custom delegate, it still works. And you can capture the evented values as a tuple.

static async void Run() {
    var result = await TaskFromEvent<int, string>(new MyClass(), "Fired");
    Console.WriteLine(result); // (123, "abcd")
}

public class MyClass {
    public delegate void TwoThings(int x, string y);

    public MyClass() {
        new Thread(() => {
            Thread.Sleep(1000);
            Fired(123, "abcd");
        }).Start();
    }

    public event TwoThings Fired;
}

Here's a helper function that'll allow you to write the TaskFromEvent functions in just one line each, if the above three methods are too much copy-and-paste for your preferences. Credit has to be given to max for simplifying what I had originally.

If you're willing to have one method per delegate type, you can do something like:

Task FromEvent(Action<Action> add)
{
    var tcs = new TaskCompletionSource<bool>();

    add(() => tcs.SetResult(true));

    return tcs.Task;
}

You would use it like:

await FromEvent(x => new MyClass().OnCompletion += x);

Be aware that this way you never unsubscribe from the event, that may or may not be a problem for you.

If you're using generic delegates, one method per each generic type is enough, you don't need one for each concrete type:

Task<T> FromEvent<T>(Action<Action<T>> add)
{
    var tcs = new TaskCompletionSource<T>();

    add(x => tcs.SetResult(x));

    return tcs.Task;
}

Although type inference doesn't work with that, you have to explicitly specify the type parameter (assuming the type of OnCompletion is Action<string> here):

string s = await FromEvent<string>(x => c.OnCompletion += x);