Change to use SetPayloadLength

src/csharp/Grpc.Core.Api/SerializationContext.cs

@@ -39,12 +39,22 @@ namespace Grpc.Core
 
         /// <summary>
         /// Gets buffer writer that can be used to write the serialized data. Once serialization is finished,
-        /// <c>Complete()</c> needs to be called.
+        /// <c>Complete()</c> needs to be called. A <c>null</c> value will be returned if serialization
+        /// with a buffer writer is not supported.
+        /// </summary>
+        public virtual IBufferWriter<byte> GetBufferWriter()
+        {
+            return null;
+        }
+
+        /// <summary>
+        /// Sets the payload length when writing serialized data a buffer writer. This method should be called before <c>GetBufferWriter</c>.
+        /// Calling this method is optional. If the payload length is not set then the length is calculated using the data written to the
+        /// buffer writer when <c>Complete()</c> is called.
         /// </summary>
         /// <param name="payloadLength">The total length of the payload in bytes.</param>
-        public virtual IBufferWriter<byte> GetBufferWriter(int payloadLength)
+        public virtual void SetPayloadLength(int payloadLength)
         {
-            throw new NotImplementedException();
         }
 
         /// <summary>

src/csharp/Grpc.Core.Tests/Internal/DefaultSerializationContextTest.cs

@@ -84,7 +84,7 @@ namespace Grpc.Core.Internal.Tests
                 var context = scope.Context;
                 var origPayload = GetTestBuffer(payloadSize);
 
-                var bufferWriter = context.GetBufferWriter(payloadSize);
+                var bufferWriter = context.GetBufferWriter();
                 origPayload.AsSpan().CopyTo(bufferWriter.GetSpan(payloadSize));
                 bufferWriter.Advance(payloadSize);
                 context.Complete();
@@ -106,7 +106,7 @@ namespace Grpc.Core.Internal.Tests
                 var context = scope.Context;
                 var origPayload = GetTestBuffer(payloadSize);
 
-                var bufferWriter = context.GetBufferWriter(payloadSize);
+                var bufferWriter = context.GetBufferWriter();
                 origPayload.AsSpan().CopyTo(bufferWriter.GetMemory(payloadSize).Span);
                 bufferWriter.Advance(payloadSize);
                 context.Complete();
@@ -131,7 +131,7 @@ namespace Grpc.Core.Internal.Tests
                 var context = scope.Context;
                 var origPayload = GetTestBuffer(payloadSize);
 
-                var bufferWriter = context.GetBufferWriter(payloadSize);
+                var bufferWriter = context.GetBufferWriter();
                 for (int offset = 0; offset < payloadSize; offset += maxSliceSize)
                 {
                     var sliceSize = Math.Min(maxSliceSize, payloadSize - offset);
@@ -165,7 +165,7 @@ namespace Grpc.Core.Internal.Tests
 
                 var origPayload2 = GetTestBuffer(20);
     
-                var bufferWriter = context.GetBufferWriter(20);
+                var bufferWriter = context.GetBufferWriter();
                 origPayload2.AsSpan().CopyTo(bufferWriter.GetMemory(origPayload2.Length).Span);
                 bufferWriter.Advance(origPayload2.Length);
                 context.Complete();
@@ -185,7 +185,7 @@ namespace Grpc.Core.Internal.Tests
                 var context = scope.Context;
                 context.Complete(GetTestBuffer(10));
 
-                Assert.Throws(typeof(InvalidOperationException), () => context.GetBufferWriter(10));
+                Assert.Throws(typeof(InvalidOperationException), () => context.GetBufferWriter());
             }
         }
 

src/csharp/Grpc.Core/Internal/DefaultSerializationContext.cs

@@ -50,12 +50,17 @@ namespace Grpc.Core.Internal
         /// <summary>
         /// Expose serializer as buffer writer
         /// </summary>
-        public override IBufferWriter<byte> GetBufferWriter(int payloadLength)
+        public override IBufferWriter<byte> GetBufferWriter()
         {
             GrpcPreconditions.CheckState(!isComplete);
             return sliceBuffer;
         }
 
+        public override void SetPayloadLength(int payloadLength)
+        {
+            // Length is calculated using the buffer writer
+        }
+
         /// <summary>
         /// Complete the payload written so far.
         /// </summary>