Protobuf scalar default values and ByteString detail

Author: JamesNK

aspnetcore/grpc/protobuf.md

@@ -69,6 +69,10 @@ Protobuf supports a range of native scalar value types. The following table list
 | `string`      | `string`     |
 | `bytes`       | `ByteString` |
 
+Scalar values always have a default value and can't be set to `null`. This constraint includes `string` and `ByteString` which are C# classes. `string` defaults to an empty string value and `ByteString` defaults to an empty bytes value. Attempting to set them to `null` throws an error.
+
+[Nullable wrapper types](#nullable-types) can be used to support null values.
+
 ### Dates and times
 
 The native scalar types don't provide for date and time values, equivalent to .NET's <xref:System.DateTimeOffset>, <xref:System.DateTime>, and <xref:System.TimeSpan>. These types can be specified by using some of Protobuf's *Well-Known Types* extensions. These extensions provide code generation and runtime support for complex field types across the supported platforms.
@@ -129,19 +133,42 @@ message Person {
 }
 ```
 
-Protobuf uses .NET nullable types, for example, `int?`, for the generated message property.
+`wrappers.proto` types aren't exposed in generated properties. Protobuf automatically maps them to appropriate .NET nullable types in C# messages. For example, a `google.protobuf.Int32Value` field generates an `int?` property. Reference type properties like `string` and `ByteString` are unchanged except `null` can be assigned to them without error.
 
 The following table shows the complete list of wrapper types with their equivalent C# type:
 
-| C# type   | Well-Known Type wrapper       |
-| --------- | ----------------------------- |
-| `bool?`   | `google.protobuf.BoolValue`   |
-| `double?` | `google.protobuf.DoubleValue` |
-| `float?`  | `google.protobuf.FloatValue`  |
-| `int?`    | `google.protobuf.Int32Value`  |
-| `long?`   | `google.protobuf.Int64Value`  |
-| `uint?`   | `google.protobuf.UInt32Value` |
-| `ulong?`  | `google.protobuf.UInt64Value` |
+| C# type      | Well-Known Type wrapper       |
+| ------------ | ----------------------------- |
+| `bool?`      | `google.protobuf.BoolValue`   |
+| `double?`    | `google.protobuf.DoubleValue` |
+| `float?`     | `google.protobuf.FloatValue`  |
+| `int?`       | `google.protobuf.Int32Value`  |
+| `long?`      | `google.protobuf.Int64Value`  |
+| `uint?`      | `google.protobuf.UInt32Value` |
+| `ulong?`     | `google.protobuf.UInt64Value` |
+| `string`     | `google.protobuf.StringValue` |
+| `ByteString` | `google.protobuf.BytesValue`  |
+
+### Bytes
+
+Binary payloads are supported in Protobuf with the `bytes` scalar value type. A generated property in C# uses `ByteString` as the property type.
+
+Use `ByteString.CopyFrom(byte[] data)` to create a new instance from a byte array:
+
+```csharp
+var data = await File.ReadAllBytesAsync(path);
+
+var payload = new PayloadResponse();
+payload.Data = ByteString.CopyFrom(data);
+```
+
+`ByteString` data is accessed directly using `ByteString.Span` or `ByteString.Memory`. Or call `ByteString.ToByteArray()` to convert an instance back into a byte array:
+
+```csharp
+var payload = await client.GetPayload(new PayloadRequest());
+
+await File.WriteAllBytesAsync(path, payload.Data.ToByteArray());
+```
 
 ### Decimals