S7.NET读写西门子字符串处理

由于西门子字符串存储跟C#字符串的存储格式不一样，在与西门子通讯时，在解析/编码字符串时需要特殊处理。在没有看到S7.NET开源库前，一直都在琢磨怎么处理好，也一直没有想到很合理的解决办法，只能怪自己的水平不行。直到看到S7.NET采用特性的办法来处理，才眼前一亮。

S7.NET字符串的特性代码如下：

using System;

namespace S7.Net.Types
{
    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
    public sealed class S7StringAttribute : Attribute
    {
        private readonly S7StringType type;
        private readonly int reservedLength;

        /// <summary>
        /// Initializes a new instance of the <see cref="S7StringAttribute"/> class.
        /// </summary>
        /// <param name="type">The string type.</param>
        /// <param name="reservedLength">Reserved length of the string in characters.</param>
        /// <exception cref="ArgumentException">Please use a valid value for the string type</exception>
        public S7StringAttribute(S7StringType type, int reservedLength)
        {
            if (!Enum.IsDefined(typeof(S7StringType), type))
                throw new ArgumentException("Please use a valid value for the string type");

            this.type = type;
            this.reservedLength = reservedLength;
        }

        /// <summary>
        /// Gets the type of the string.
        /// </summary>
        /// <value>
        /// The string type.
        /// </value>
        public S7StringType Type => type;

        /// <summary>
        /// Gets the reserved length of the string in characters.
        /// </summary>
        /// <value>
        /// The reserved length of the string in characters.
        /// </value>
        public int ReservedLength => reservedLength;

        /// <summary>
        /// Gets the reserved length in bytes.
        /// </summary>
        /// <value>
        /// The reserved length in bytes.
        /// </value>
        public int ReservedLengthInBytes => type == S7StringType.S7String ? reservedLength + 2 : (reservedLength * 2) + 4;
    }


    /// <summary>
    /// String type.
    /// </summary>
    public enum S7StringType
    {
        /// <summary>
        /// ASCII string.
        /// </summary>
        S7String = VarType.S7String,

        /// <summary>
        /// Unicode string.
        /// </summary>
        S7WString = VarType.S7WString
    }
}

解码与编码字符串代码如下：

using System;
using System.Text;

namespace S7.Net.Types
{
    /// <summary>
    /// Contains the methods to convert from S7 strings to C# strings
    /// An S7 String has a preceeding 2 byte header containing its capacity and length
    /// </summary>
    public static class S7String
    {
        private static Encoding stringEncoding = Encoding.ASCII;

        /// <summary>
        /// The Encoding used when serializing and deserializing S7String (Encoding.ASCII by default)
        /// </summary>
        /// <exception cref="ArgumentNullException">StringEncoding must not be null</exception>
        public static Encoding StringEncoding
        {
            get => stringEncoding;
            set => stringEncoding = value ?? throw new ArgumentNullException(nameof(StringEncoding));
        }

        /// <summary>
        /// Converts S7 bytes to a string
        /// </summary>
        /// <param name="bytes"></param>
        /// <returns></returns>
        public static string FromByteArray(byte[] bytes)
        {
            if (bytes.Length < 2)
            {
                throw new PlcException(ErrorCode.ReadData, "Malformed S7 String / too short");
            }

            int size = bytes[0];
            int length = bytes[1];
            if (length > size)
            {
                throw new PlcException(ErrorCode.ReadData, "Malformed S7 String / length larger than capacity");
            }

            try
            {
                return StringEncoding.GetString(bytes, 2, length);
            }
            catch (Exception e)
            {
                throw new PlcException(ErrorCode.ReadData,
                    $"Failed to parse {VarType.S7String} from data. Following fields were read: size: '{size}', actual length: '{length}', total number of bytes (including header): '{bytes.Length}'.",
                    e);
            }
        }

        /// <summary>
        /// Converts a <see cref="T:string"/> to S7 string with 2-byte header.
        /// </summary>
        /// <param name="value">The string to convert to byte array.</param>
        /// <param name="reservedLength">The length (in characters) allocated in PLC for the string.</param>
        /// <returns>A <see cref="T:byte[]" /> containing the string header and string value with a maximum length of <paramref name="reservedLength"/> + 2.</returns>
        public static byte[] ToByteArray(string value, int reservedLength)
        {
            if (value is null)
            {
                throw new ArgumentNullException(nameof(value));
            }

            if (reservedLength > 254) throw new ArgumentException($"The maximum string length supported is 254.");

            var bytes = StringEncoding.GetBytes(value);
            if (bytes.Length > reservedLength) throw new ArgumentException($"The provided string length ({bytes.Length} is larger than the specified reserved length ({reservedLength}).");

            var buffer = new byte[2 + reservedLength];
            Array.Copy(bytes, 0, buffer, 2, bytes.Length);
            buffer[0] = (byte)reservedLength;
            buffer[1] = (byte)bytes.Length;
            return buffer;
        }
    }
}

在处理Class中使用方法如下：

using System;
using System.Text;

namespace S7.Net.Types
{
    /// <summary>
    /// Contains the methods to convert from S7 strings to C# strings
    /// An S7 String has a preceeding 2 byte header containing its capacity and length
    /// </summary>
    public static class S7String
    {
        private static Encoding stringEncoding = Encoding.ASCII;

        /// <summary>
        /// The Encoding used when serializing and deserializing S7String (Encoding.ASCII by default)
        /// </summary>
        /// <exception cref="ArgumentNullException">StringEncoding must not be null</exception>
        public static Encoding StringEncoding
        {
            get => stringEncoding;
            set => stringEncoding = value ?? throw new ArgumentNullException(nameof(StringEncoding));
        }

        /// <summary>
        /// Converts S7 bytes to a string
        /// </summary>
        /// <param name="bytes"></param>
        /// <returns></returns>
        public static string FromByteArray(byte[] bytes)
        {
            if (bytes.Length < 2)
            {
                throw new PlcException(ErrorCode.ReadData, "Malformed S7 String / too short");
            }

            int size = bytes[0];
            int length = bytes[1];
            if (length > size)
            {
                throw new PlcException(ErrorCode.ReadData, "Malformed S7 String / length larger than capacity");
            }

            try
            {
                return StringEncoding.GetString(bytes, 2, length);
            }
            catch (Exception e)
            {
                throw new PlcException(ErrorCode.ReadData,
                    $"Failed to parse {VarType.S7String} from data. Following fields were read: size: '{size}', actual length: '{length}', total number of bytes (including header): '{bytes.Length}'.",
                    e);
            }
        }

        /// <summary>
        /// Converts a <see cref="T:string"/> to S7 string with 2-byte header.
        /// </summary>
        /// <param name="value">The string to convert to byte array.</param>
        /// <param name="reservedLength">The length (in characters) allocated in PLC for the string.</param>
        /// <returns>A <see cref="T:byte[]" /> containing the string header and string value with a maximum length of <paramref name="reservedLength"/> + 2.</returns>
        public static byte[] ToByteArray(string value, int reservedLength)
        {
            if (value is null)
            {
                throw new ArgumentNullException(nameof(value));
            }

            if (reservedLength > 254) throw new ArgumentException($"The maximum string length supported is 254.");

            var bytes = StringEncoding.GetBytes(value);
            if (bytes.Length > reservedLength) throw new ArgumentException($"The provided string length ({bytes.Length} is larger than the specified reserved length ({reservedLength}).");

            var buffer = new byte[2 + reservedLength];
            Array.Copy(bytes, 0, buffer, 2, bytes.Length);
            buffer[0] = (byte)reservedLength;
            buffer[1] = (byte)bytes.Length;
            return buffer;
        }
    }
}

字符串使用方式,使用特性指定字符串长度：

        [S7String(S7StringType.S7WString, 10)] 
        public string WStringVariable;


        [S7String(S7StringType.S7String, 10)]
        public string StringVariable;

大家如果有更好的办法，欢迎评论！