rework the how-to for casting and pattern matching

.openpublishing.redirection.json

@@ -1446,6 +1446,10 @@
             "source_path":"docs/csharp/programming-guide/nullable-types/how-to-safely-cast-from-bool-to-bool.md",
             "redirect_url":"/dotnet/csharp/programming-guide/nullable-types/using-nullable-types"
         },
+        {
+            "source_path":"docs/csharp/programming-guide/types/how-to-safely-cast-using-as-and-is-operators.md",
+            "redirect_url":"/dotnet/csharp/how-to/safely-cast-using-pattern-matching-is-and-as-operators"
+        },
         {
             "source_path":"docs/standard/asynchronous-programming-patterns/multithreaded-programming-with-the-event-based-asynchronous-pattern.md",
             "redirect_url":"/dotnet/standard/asynchronous-programming-patterns/event-based-asynchronous-pattern-eap"

docs/csharp/how-to/index.md

@@ -63,7 +63,7 @@ You may need to convert an object to a different type.
 - [Convert a string to a `DateTime`](../../standard/base-types/parsing-datetime.md).
 - [Convert a byte array to an int](../programming-guide/types/how-to-convert-a-byte-array-to-an-int.md).
 - [Convert a string to a number](../programming-guide/types/how-to-convert-a-string-to-a-number.md).
-- [Use `as` and `is` to safely cast to a different type](../programming-guide/types/how-to-safely-cast-by-using-as-and-is-operators.md).
+- [Use pattern matching, the `as` and `is` operators to safely cast to a different type](../how-to/safely-cast-using-pattern-matching-is-and-as-operators.md).
 - [Define conversion operators for `struct` types](../programming-guide/statements-expressions-operators/how-to-implement-user-defined-conversions-between-structs.md).
 - [Determine if a type is a nullable value type](../programming-guide/nullable-types/how-to-identify-a-nullable-type.md).
 - [Convert between nullable and non-nullable value types](../programming-guide/nullable-types/using-nullable-types.md#conversion-from-a-nullable-type-to-an-underlying-type).

docs/csharp/how-to/safely-cast-using-pattern-matching-is-and-as-operators.md

@@ -0,0 +1,36 @@
+---
+title: "How to: safely cast by using pattern matching and the is and as operators"
+description: Learn to use pattern matching techniques to safely cast variables to a different type. You can use pattern matching as well as the is and as operators to safely convert types.
+ms.date: 09/05/2018
+helpviewer_keywords: 
+  - "cast operators [C#], as and is operators"
+  - "as operator [C#]"
+  - "is operator [C#]"
+---
+# How to: safely cast by using pattern matching is and as operators
+
+Because objects are polymorphic, it's possible for a variable of a base class type to hold a derived [type](../programming-guide/types/index.md). To access the derived type's instance members, it's necessary to [cast](../programming-guide/types/casting-and-type-conversions.md) the value back to the derived type. However, a cast creates the risk of throwing an <xref:System.InvalidCastException>. C# provides [pattern matching](../pattern-matching.md) statements that perform a cast conditionally only when it will succeed. C# also provides the [is](../language-reference/keywords/is.md) and [as](../language-reference/keywords/as.md) operators to test if a value is of a certain type.
+
+[!INCLUDE[interactive-note](~/includes/csharp-interactive-note.md)]
+
+The following code demonstrates the pattern matching `is` statement. It contains methods that test a method argument to determine if it is one of a possible set of derived types:
+
+[!code-csharp-interactive[Pattern matching is statement](../../../samples/snippets/csharp/how-to/safelycast/patternmatching/Program.cs#PatternMatchingIs)]
+
+The preceding sample demonstrates a few features of pattern matching syntax. The `if (a is Mammal m)` and `if (o is Mammal m)` statements combine the test with an initialization assignment. TThe assignment occurs only when the test succeeds. The variable `m` is only in scope in the embedded `if` statement where it has been assigned. You cannot access `m` later in the same method. Try it in the interactive window.
+
+You can also use the same syntax for testing if a [nullable type](../programming-guide/nullable-types/index.md) has a value, as shown in the following sample code:
+
+[!code-csharp-interactive[Pattern matching with nullable types](../../../samples/snippets/csharp/how-to/safelycast/nullablepatternmatching/Program.cs#PatternMatchingNullable)]
+
+The preceding sample demonstrates other features of pattern matching to use with conversions. You can test a variable for the null pattern by checking specifically for the `null` value. When the runtime value of the variable is `null`, an `is` statement checking for a type always returns `false`. The pattern matching `is` statement doesn't allow a nullable value type, such as `int?` or `Nullable<int>`, but you can test for any other value type.
+
+The preceding sample also shows how you use the pattern matching `is` expression in a `switch` statement where the variable may be one of many different types.
+
+If you want to test if a variable is a given type, but not assign it to a new variable, you can use the `is` and `as` operators for reference types and nullable types. The following code shows how to use the `is` and `as` statements that were part of the C# language before pattern matching was introduced to test if a variable is of a given type:
+
+[!code-csharp-interactive[testing variable types with the is and as statements](../../../samples/snippets/csharp/how-to/safelycast/asandis/Program.cs#IsAndAs)]
+
+As you can see by comparing this code with the pattern matching code, the pattern matching syntax provides more robust features by combining the test and the assignment in a single statement. Use the pattern matching syntax whenever possible.
+
+You can try these samples by looking at the code in our [GitHub repository](https://github.com/dotnet/samples/tree/master/snippets/csharp/how-to/safelycast). Or you can download the samples [as a zip file](https://github.com/dotnet/samples/raw/master/snippets/csharp/how-to/safelycast.zip).

docs/csharp/language-reference/keywords/dynamic.md

@@ -66,5 +66,5 @@ For more information and examples, see [Using Type dynamic](../../../csharp/prog
 - [is](../../../csharp/language-reference/keywords/is.md)  
 - [as](../../../csharp/language-reference/keywords/as.md)  
 - [typeof](../../../csharp/language-reference/keywords/typeof.md)  
-- [How to: Safely Cast by Using as and is Operators](../../../csharp/programming-guide/types/how-to-safely-cast-by-using-as-and-is-operators.md)  
+- [How to: Safely cast Using pattern matching, as, and is Operators](../../how-to/safely-cast-using-pattern-matching-is-and-as-operators.md)  
 - [Walkthrough: Creating and Using Dynamic Objects](../../../csharp/programming-guide/types/walkthrough-creating-and-using-dynamic-objects.md)

docs/csharp/programming-guide/types/casting-and-type-conversions.md

@@ -73,7 +73,7 @@ Giraffe g2 = (Giraffe) a;
 
  [!code-csharp[csProgGuideTypes#41](../../../csharp/programming-guide/nullable-types/codesnippet/CSharp/casting-and-type-conversions_3.cs)]  
 
- C# provides the [is](../../../csharp/language-reference/keywords/is.md) and [as](../../../csharp/language-reference/keywords/as.md) operators to enable you to test for compatibility before actually performing a cast. For more information, see [How to: Safely Cast by Using as and is Operators](../../../csharp/programming-guide/types/how-to-safely-cast-by-using-as-and-is-operators.md).  
+ C# provides the [is](../../../csharp/language-reference/keywords/is.md) and [as](../../../csharp/language-reference/keywords/as.md) operators to enable you to test for compatibility before actually performing a cast. For more information, see [How to: Safely cast using pattern matching, as and is Operators](../../how-to/safely-cast-using-pattern-matching-is-and-as-operators.md).  
 
 ## C# Language Specification  
  [!INCLUDE[CSharplangspec](~/includes/csharplangspec-md.md)]  

docs/csharp/programming-guide/types/toc.md

@@ -3,7 +3,6 @@
 ## [Boxing and Unboxing](boxing-and-unboxing.md)
 ## [Using Type dynamic](using-type-dynamic.md)
 ### [Walkthrough: Creating and Using Dynamic Objects (C# and Visual Basic)](walkthrough-creating-and-using-dynamic-objects.md)
-## [How to: Safely Cast by Using as and is Operators](how-to-safely-cast-by-using-as-and-is-operators.md)
 ## [How to: Convert a byte Array to an int](how-to-convert-a-byte-array-to-an-int.md)
 ## [How to: Convert a String to a Number](how-to-convert-a-string-to-a-number.md)
 ## [How to: Convert Between Hexadecimal Strings and Numeric Types](how-to-convert-between-hexadecimal-strings-and-numeric-types.md)

docs/toc.md

@@ -288,6 +288,7 @@
 ### [Search strings](csharp/how-to/search-strings.md)
 ### [Modify string contents](csharp/how-to/modify-string-contents.md)
 ### [Compare strings](csharp/how-to/compare-strings.md)
+### [Safely cast using pattern matching, is and as operators](csharp/how-to/safely-cast-using-pattern-matching-is-and-as-operators.md)
 <!-- End of C# Concepts section -->
 ## [The .NET Compiler Platform SDK (Roslyn APIs)](csharp/roslyn-sdk/)
 ## [C# Programming Guide](csharp/programming-guide/)