Avoid NullReferenceException with async methods

When using async methods and loose Mock Behavior, calls to Task methods
give NullReferenceException, since default(Task) is null. Calls to
Task.Wait() and Task<T>.Result should follow the loose behavior: do
nothing and return the default value for T, respectively.

Related to devlooped#64.

Source/EmptyDefaultValueProvider.cs

@@ -43,6 +43,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
+using System.Threading.Tasks;
 
 namespace Moq
 {
@@ -86,6 +87,11 @@ private static object GetReferenceTypeDefault(Type valueType)
 			{
 				return new object[0].AsQueryable();
 			}
+			else if (valueType == typeof(Task))
+			{
+				// Task<T> inherits from Task, so just return Task<bool>
+				return GetCompletedTaskWithResult(false);
+			}
 			else if (valueType.IsGenericType && valueType.GetGenericTypeDefinition() == typeof(IEnumerable<>))
 			{
 				var genericListType = typeof(List<>).MakeGenericType(valueType.GetGenericArguments()[0]);
@@ -101,6 +107,13 @@ private static object GetReferenceTypeDefault(Type valueType)
 					.MakeGenericMethod(genericType)
 					.Invoke(null, new[] { Activator.CreateInstance(genericListType) });
 			}
+			else if (valueType.IsGenericType && valueType.GetGenericTypeDefinition() == typeof(Task<>))
+			{
+				var genericType = valueType.GetGenericArguments()[0];
+
+				return GetCompletedTaskWithResult(
+					genericType.IsValueType ? GetValueTypeDefault(genericType) : GetReferenceTypeDefault(genericType));
+			}
 
 			return null;
 		}
@@ -115,5 +128,17 @@ private static object GetValueTypeDefault(Type valueType)
 
 			return Activator.CreateInstance(valueType);
 		}
+
+		private static Task GetCompletedTaskWithResult<TResult>(TResult value)
+		{
+			var type = value.GetType();
+			var tcs = Activator.CreateInstance(typeof (TaskCompletionSource<>).MakeGenericType(type));
+
+			var setResultMethod = tcs.GetType().GetMethod("SetResult");
+			var taskProperty = tcs.GetType().GetProperty("Task");
+
+			setResultMethod.Invoke(tcs, new object[] {value});
+			return (Task) taskProperty.GetValue(tcs, null);
+		}
 	}
 }

UnitTests/EmptyDefaultValueProviderFixture.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Moq.Tests
@@ -107,6 +108,41 @@ public void ProvideEmptyQueryableObjects()
 			Assert.Equal(0, ((IQueryable)value).Cast<object>().Count());
 		}
 
+		[Fact]
+		public void ProvidesDefaultTask()
+		{
+			var provider = new EmptyDefaultValueProvider();
+
+			var value = provider.ProvideDefault(typeof(IFoo).GetProperty("TaskValue").GetGetMethod());
+
+			Assert.NotNull(value);
+			Assert.True(((Task)value).IsCompleted);
+		}
+
+		[Fact]
+		public void ProvidesDefaultGenericTask()
+		{
+			var provider = new EmptyDefaultValueProvider();
+
+			var value = provider.ProvideDefault(typeof(IFoo).GetProperty("GenericTaskValue").GetGetMethod());
+
+			Assert.NotNull(value);
+			Assert.True(((Task)value).IsCompleted);
+			Assert.Equal(default(int), ((Task<int>)value).Result);
+		}
+
+		[Fact]
+		public void ProvidesDefaultTaskOfGenericTask()
+		{
+			var provider = new EmptyDefaultValueProvider();
+
+			var value = provider.ProvideDefault(typeof(IFoo).GetProperty("TaskOfGenericTaskValue").GetGetMethod());
+
+			Assert.NotNull(value);
+			Assert.True(((Task)value).IsCompleted);
+			Assert.Equal(default(int), ((Task<Task<int>>) value).Result.Result);
+		}
+
 		public interface IFoo
 		{
 			object Object { get; set; }
@@ -120,6 +156,9 @@ public interface IFoo
 			IBar[] Bars { get; set; }
 			IQueryable<int> Queryable { get; }
 			IQueryable QueryableObjects { get; }
+			Task TaskValue { get; set; }
+			Task<int> GenericTaskValue { get; set; }
+			Task<Task<int>> TaskOfGenericTaskValue { get; set; }
 		}
 
 		public interface IBar { }