ASP.NET 以流的方式读取 MSSQL text、ntext 或 image 字段值

类中用到SQL中的 DATALENGTH 函数 和READTEXT 存储过程 都是系统内置 ，有关信息请搜索一下便知

 

using System;
using System.Data;
using System.Data.SqlClient;
using System.IO;

public sealed class SqlClientOutputStream : Stream
{
    private SqlConnection _conn;
    private long _datasize; //要读取数据的字节数
    private SqlCommand _com;
    private SqlParameter _sqlpar;
    private long _datalength; //数据所占的字节数
    private SqlParameter _sqlpar2;

    public SqlClientOutputStream(string connectionString, string table, string dataField, string whereCriteria)
    {
        byte[] buffer;
        this._conn = new SqlConnection(connectionString);
        this._com = this._conn.CreateCommand();
        this._com.CommandText = "SELECT DATALENGTH(" + dataField + ") FROM " + table + " WHERE " + whereCriteria;
        try
        {
            this._conn.Open();
            this._datalength = (int)this._com.ExecuteScalar();
        }
        finally
        {
            this._conn.Close();
        }
        this._com.CommandText = "SELECT TEXTPTR(" + dataField + ") FROM " + table + " WHERE " + whereCriteria;
        try
        {
            this._conn.Open();
            buffer = (byte[])this._com.ExecuteScalar();
        }
        finally
        {
            this._conn.Close();
        }
        this._com.CommandText = "READTEXT " + table + "." + dataField + " @ptr @offset @size;";
        SqlParameter parameter = this._com.CreateParameter();
        parameter.DbType = DbType.Binary;
        parameter.ParameterName = "@ptr";
        parameter.Size = 0x10;
        parameter.Value = buffer;
        this._com.Parameters.Add(parameter);
        this._sqlpar2 = this._com.CreateParameter();
        this._sqlpar2.DbType = DbType.Int32;
        this._sqlpar2.ParameterName = "@offset";
        this._sqlpar2.Size = 4;
        this._com.Parameters.Add(this._sqlpar2);
        this._sqlpar = this._com.CreateParameter();
        this._sqlpar.DbType = DbType.Int32;
        this._sqlpar.ParameterName = "@size";
        this._sqlpar.Size = 4;
        this._com.Parameters.Add(this._sqlpar);
    }

    public SqlClientOutputStream(string connectionString, string table, string dataField, string idField, int idValue)
        : this(connectionString, table, dataField, idField + "=" + idValue.ToString())
    {
    }

    public override void Close()
    {
        base.Close();
        this._conn.Dispose();
    }

    public override void Flush()
    {
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        int num;
        if (this._datasize > this._datalength)
        {
            throw new InvalidOperationException("Tried to read past end of stream.");
        }
        if (this._datasize == this._datalength)
        {
            return 0;
        }
        if ((this._datasize + count) > this._datalength)
        {
            num = (int)(this._datalength - this._datasize);
        }
        else
        {
            num = count;
        }
        this._sqlpar.Value = num;
        this._sqlpar2.Value = this._datasize;
        byte[] src = null;
        try
        {
            this._conn.Open();
            src = (byte[])this._com.ExecuteScalar();
        }
        finally
        {
            this._conn.Close();
        }
        Buffer.BlockCopy(src, 0, buffer, offset, num);
        this._datasize += num;
        return num;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        switch (origin)
        {
            case SeekOrigin.Begin:
                if ((offset < 0L) || (offset > this._datalength))
                {
                    throw new ArgumentOutOfRangeException("offset");
                }
                this._datasize = offset;
                break;

            case SeekOrigin.Current:
                if (((this._datasize + offset) < 0L) || ((this._datasize + offset) > this._datalength))
                {
                    throw new ArgumentOutOfRangeException("offset");
                }
                this._datasize += offset;
                break;

            case SeekOrigin.End:
                if ((offset > 0L) || ((this._datalength + offset) < 0L))
                {
                    throw new ArgumentOutOfRangeException("offset");
                }
                this._datasize = this._datalength + offset;
                break;
        }
        return this._datasize;
    }

    public override void SetLength(long value)
    {
        throw new NotSupportedException();
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        throw new NotSupportedException();
    }

    public override bool CanRead
    {
        get
        {
            return true;
        }
    }

    public override bool CanSeek
    {
        get
        {
            return true;
        }
    }

    public override bool CanWrite
    {
        get
        {
            return false;
        }
    }

    public override long Length
    {
        get
        {
            return this._datalength;
        }
    }

    public override long Position
    {
        get
        {
            return this._datasize;
        }
        set
        {
            if ((this._datasize < 0L) || (this._datasize > this._datalength))
            {
                throw new ArgumentOutOfRangeException("value");
            }
            this._datasize = value;
        }
    }
}