Disallow open generics on activity methods (#354)

Fixes #350

Author: cretz

src/Temporalio/Activities/ActivityDefinition.cs

@@ -126,6 +126,10 @@ public static ActivityDefinition Create(MethodInfo method, Func<object?[], objec
         {
             var attr = method.GetCustomAttribute<ActivityAttribute>(false) ??
                 throw new ArgumentException($"{method} missing Activity attribute");
+            if (method.ContainsGenericParameters)
+            {
+                throw new ArgumentException($"{method} contains generic parameters");
+            }
             var parms = method.GetParameters();
             return Create(
                 NameFromAttributed(method, attr),

tests/Temporalio.Tests/Activities/ActivityDefinitionTests.cs

@@ -156,6 +156,23 @@ public async Task CreateAll_ClassOfStaticActivities_CanInvoke()
         Assert.Equal(128, await defn.InvokeAsync(new object?[] { 123 }));
     }
 
+    [Fact]
+    public void CreateAll_OpenGeneric_Throws()
+    {
+        var exc = Assert.ThrowsAny<Exception>(() => ActivityDefinition.CreateAll(
+            typeof(BadActivityGeneric), new BadActivityGeneric()));
+        Assert.Contains("contains generic parameters", exc.Message);
+    }
+
+    [Fact]
+    public async Task CreateAll_ClosedGeneric_CanInvoke()
+    {
+        var defn = ActivityDefinition.CreateAll(
+            typeof(GoodActivityGeneric<string>),
+            new GoodActivityGeneric<string>("some-val")).Single();
+        Assert.Equal("some-val", await defn.InvokeAsync(Array.Empty<object?>()));
+    }
+
     protected static void BadAct1()
     {
     }
@@ -174,6 +191,22 @@ public static class GoodActivityClassStatic
         public static int MyActivity(int param) => param + 5;
     }
 
+    public class BadActivityGeneric
+    {
+        [Activity]
+        public T BadActAsync<T>() => throw new NotSupportedException();
+    }
+
+    public class GoodActivityGeneric<T>
+    {
+        private readonly T result;
+
+        public GoodActivityGeneric(T result) => this.result = result;
+
+        [Activity]
+        public T GoodAsyncAsync() => result;
+    }
+
     public class BadActivityClassNoActivities
     {
         public int ActivityWithoutAttribute(int param) => param + 5;