Implement if(let ...) syntax (#3673) (#3958)

1 files changed, 58 insertions, 0 deletions

diff --git a/docs/user-guide/06-interfaces-generics.md b/docs/user-guide/06-interfaces-generics.md
index ba543df56..cb6a5070e 100644
--- a/docs/user-guide/06-interfaces-generics.md
+++ b/docs/user-guide/06-interfaces-generics.md
@@ -677,6 +677,64 @@ T compute<T>(T a1, T a2)
 // compute(3, 1) == 2
 ```
 
+`as` operator can also be used in the `if` predicate to test if an object can be casted to a specific type, once the cast test is successful,
+the object can be used in the `if` block as the casted type without the need to retrieve the `Optional<T>::value` property:
+```csharp
+interface IFoo
+{
+    void foo();
+}
+
+struct MyImpl1 : IFoo
+{
+    void foo() { printf("MyImpl1");}
+}
+
+struct MyImpl2 : IFoo
+{
+    void foo() { printf("MyImpl2");}
+}
+
+struct MyImpl3 : IFoo
+{
+    void foo() { printf("MyImpl3");}
+}
+
+void test(IFoo foo)
+{
+    // This syntax will be desugared to the following:
+    // {
+    //      Optional<MyImpl1> $OptVar = foo as MyImpl1;
+    //      if ($OptVar.hasValue)
+    //      {
+    //          MyImpl1 t = $OptVar.value;
+    //          t.foo();
+    //      }
+    //      else if ...
+    // }
+    if (let t = foo as MyImpl1) // t is of type MyImpl1
+    {
+        t.foo();
+    }
+    else if (let t = foo as MyImpl2) // t is of type MyImpl2
+    {
+        t.foo();
+    }
+    else
+        printf("fail");
+}
+
+void main()
+{
+    MyImpl1 v1;
+    test(v1);
+
+    MyImpl2 v2;
+    test(v2);
+}
+
+```
+
 Extensions to Interfaces
 -----------------------------