C#调用Dll传递字符串指针参数.

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最近用c#写一个小软件，要调用VC写的dll,传递参数给dll,由于是非托管代码，所以调用起来有点麻烦，要把参数转换成非托管代码的指针参数，然后再传递给dll

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using System.Runtime.InteropServices; //操作Dll的类空间
class CGicomIndex
{
[DllImport("Index_dll.dll")]
unsafe private static extern Int32 create_index_file(IntPtr filename, IntPtr fieldname, UInt32 tongshu, IntPtr err);
//VC原型　create_index_file( char*filename, char * fieldname, unsigned long tongshu, char *err);
#region " bool CreateIndexFile( string m_strFileName, string m_strFieldName, UInt32 m_key, ref string strErr ) 对DBF文件按照指定的字段创建索引"
/// <summary>
/// 对DBF文件按照指定的字段创建索引
/// </summary>
/// <param name="m_strFileName">DBF文件路径</param>
/// <param name="m_strFieldName">创建索引的字段名称</param>
/// <param name="m_key">索引桶数(文件记录的最大数)</param>
/// <param name="strErr">出错信息</param>
/// <returns>成功 true</returns>
public static bool CreateIndexFile( string m_strFileName, string m_strFieldName, UInt32 m_key, ref string strErr )
{
try
{
IntPtr ptrFileName, ptrFieldName, ptrErr;
//根据数据的长度申请非托管空间
ptrFileName = mallocIntptr(m_strFileName);
ptrFieldName = mallocIntptr(m_strFieldName);
ptrErr = mallocIntptr(300);
//创建索引
if (create_index_file(ptrFileName, ptrFieldName, m_key, ptrErr) != 1)
{
strErr = Marshal.PtrToStringAnsi(ptrErr);
Marshal.FreeHGlobal(ptrFileName);
Marshal.FreeHGlobal(ptrFieldName);
Marshal.FreeHGlobal(ptrErr);
return false;
}
Marshal.FreeHGlobal(ptrFileName);
Marshal.FreeHGlobal(ptrFieldName);
Marshal.FreeHGlobal(ptrErr);
return true;
}
catch (System.Exception ex)
{
strErr = ex.Message;
return false;
}
}
#endregion
#region " IntPtr mallocIntptr( string strData ) 根据数据的长度申请非托管空间"
/// <summary>
/// 根据数据的长度申请非托管空间
/// </summary>
/// <param name="strData">要申请非托管空间的数据</param>
/// <returns>指向非拖管空间的指针</returns>
private static IntPtr mallocIntptr( string strData )
{
//先将字符串转化成字节方式
Byte[] btData = System.Text.Encoding.Default.GetBytes(strData);
//申请非拖管空间
IntPtr m_ptr = Marshal.AllocHGlobal(btData.Length);
//给非拖管空间清０
Byte[] btZero = new Byte[btData .Length+ 1]; //一定要加1,否则后面是乱码，原因未找到
Marshal.Copy(btZero, 0, m_ptr, btZero.Length);
//给指针指向的空间赋值
Marshal.Copy(btData, 0, m_ptr, btData.Length);
return m_ptr;
}
/// <summary>
/// 根据长度申请非托管空间
/// </summary>
/// <param name="strData">要申请非托管空间的大小</param>
/// <returns>指向非拖管空间的指针</returns>
private static IntPtr mallocIntptr( int length )
{
//申请非拖管空间
IntPtr m_ptr = Marshal.AllocHGlobal(length);
//给非拖管空间清０
Byte[] btZero = new Byte[length + 1]; //一定要加1,否则后面是乱码，原因未找到
Marshal.Copy(btZero, 0, m_ptr, btZero.Length);
//给指针指向的空间赋值
Marshal.Copy(btZero, 0, m_ptr, length);
return m_ptr;
}
#endregion
}