Add Thrift C# runtime library

Summary:
Add C# runtime library for Thrift binary and compact protocol serialization

Introduces a production-ready C# Thrift runtime library supporting both binary and compact protocols, enabling Unity and other C# applications to serialize/deserialize Thrift data.

Key components:
- Protocol readers/writers: ThriftBinaryReader/Writer and ThriftCompactReader/Writer
- Protocol-agnostic interfaces: IThriftProtocolReader/Writer for polymorphic usage
- Core types: IThriftSerializable interface, ThriftWireType enum, ThriftProtocolException
- Helper utilities: ThriftProtocolHelper with Skip(), ReadValue<T>(), WriteValue<T>() for shared logic

Implementation details:
- Binary protocol: Big-endian encoding for all multi-byte values
- Compact protocol: Varint encoding, zigzag for signed integers, delta field IDs, inline booleans, compressed collection headers
- Security: Configurable string size limits, nesting depth protection (64 max), strict UTF-8 validation
- Testing: Comprehensive protocol-agnostic round-trip tests plus format-specific wire encoding tests

Mirrored via dirsync between xplat/thrift/lib/csharp/ and fbcode/thrift/lib/csharp/ to support both build environments.

Reviewed By: vitaut

Differential Revision: D94160930

fbshipit-source-id: 21289b4bad3aaecec2ce5a53526b8d3dbb7bd47f

Author: abrady

third-party/thrift/src/thrift/lib/csharp/IThriftProtocolReader.cs

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System.Collections.Generic;
+
+namespace FBThrift
+{
+    /// <summary>
+    /// Protocol-agnostic interface for reading Thrift data.
+    /// Implemented by ThriftBinaryReader and ThriftCompactReader.
+    /// </summary>
+    public interface IThriftProtocolReader
+    {
+        /// <summary>
+        /// Gets or sets the maximum allowed size for string and binary reads.
+        /// When set to 0 (default), no configurable limit is enforced, but reads
+        /// are still validated against remaining stream bytes.
+        /// </summary>
+        int StringSizeLimit { get; set; }
+
+        (ThriftWireType fieldType, short fieldId) ReadFieldBegin();
+        bool ReadBool();
+        sbyte ReadByte();
+        short ReadI16();
+        int ReadI32();
+        long ReadI64();
+        float ReadFloat();
+        double ReadDouble();
+        string ReadString();
+        byte[] ReadBinary();
+        (ThriftWireType elemType, int size) ReadListBegin();
+        (ThriftWireType elemType, int size) ReadSetBegin();
+        (ThriftWireType keyType, ThriftWireType valType, int size) ReadMapBegin();
+        T ReadStruct<T>() where T : IThriftSerializable, new();
+        HashSet<T> ReadSet<T>();
+        List<T> ReadList<T>();
+        Dictionary<K, V> ReadMap<K, V>();
+        void Skip(ThriftWireType fieldType, short? fieldId = null);
+        T ReadValue<T>();
+    }
+}

third-party/thrift/src/thrift/lib/csharp/IThriftProtocolWriter.cs

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System;
+using System.Collections.Generic;
+
+namespace FBThrift
+{
+    /// <summary>
+    /// Protocol-agnostic interface for writing Thrift data.
+    /// Implemented by ThriftBinaryWriter and ThriftCompactWriter.
+    /// </summary>
+    public interface IThriftProtocolWriter
+    {
+        void WriteFieldBegin(ThriftWireType fieldType, short fieldId);
+        void WriteFieldStop();
+        void WriteBool(bool value);
+        void WriteByte(sbyte value);
+        void WriteI16(short value);
+        void WriteI32(int value);
+        void WriteI64(long value);
+        void WriteFloat(float value);
+        void WriteDouble(double value);
+        void WriteString(string value);
+        void WriteBinary(byte[] value);
+        void WriteListBegin(ThriftWireType elemType, int size);
+        void WriteSetBegin(ThriftWireType elemType, int size);
+        void WriteMapBegin(ThriftWireType keyType, ThriftWireType valType, int size);
+        void WriteStruct(IThriftSerializable value);
+        void WriteSet<T>(IEnumerable<T> value);
+        void WriteList<T>(IEnumerable<T> value);
+        void WriteMap<K, V>(IDictionary<K, V> value);
+        void WriteValue<T>(T value);
+    }
+}

third-party/thrift/src/thrift/lib/csharp/IThriftSerializable.cs

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace FBThrift
+{
+    /// <summary>
+    /// Interface for types that can be serialized and deserialized
+    /// using any Thrift protocol (Binary, Compact, etc.).
+    /// </summary>
+    public interface IThriftSerializable
+    {
+        /// <summary>
+        /// Writes this object to the specified Thrift protocol writer.
+        /// </summary>
+        /// <param name="writer">The protocol writer to write to.</param>
+        void Write(IThriftProtocolWriter writer);
+
+        /// <summary>
+        /// Reads this object's fields from the specified Thrift protocol reader.
+        /// </summary>
+        /// <param name="reader">The protocol reader to read from.</param>
+        void Read(IThriftProtocolReader reader);
+    }
+}

third-party/thrift/src/thrift/lib/csharp/ThriftProtocolException.cs

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System;
+
+namespace FBThrift
+{
+    /// <summary>
+    /// Exception thrown when a Thrift protocol error occurs during serialization or deserialization.
+    /// </summary>
+    public class ThriftProtocolException : Exception
+    {
+        public ThriftProtocolException(string message)
+            : base(message)
+        {
+        }
+
+        public ThriftProtocolException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+    }
+}