string uriString = "http://localhost/login.aspx"; // 创建一个新的 WebClient 实例. WebClient myWebClient = new WebClient(); string postData = "Username=admin&Password=admin"; // 注意这种拼字符串的ContentType myWebClient.Headers.Add("Content-Type","application/x-www-form-urlencoded"); // 转化成二进制数组 byte[] byteArray = Encoding.ASCII.GetBytes(postData); // 上传数据，并获取返回的二进制数据. byte[] responseArray = myWebClient.UploadData(uriString,"POST",byteArray);

String uriString = "http://localhost/uploadFile.aspx"; // 创建一个新的 WebClient 实例. WebClient myWebClient = new WebClient(); string fileName = @"C:\upload.txt"; // 直接上传，并获取返回的二进制数据. byte[] responseArray = myWebClient.UploadFile(uriString,"POST",fileName);

-----------------------------7d429871607fe
Content-Disposition: form-data; name="file1"; filename="G:\homepage.txt"
Content-Type: text/plain
宝玉：http://www.webuc.net
-----------------------------7d429871607fe
Content-Disposition: form-data; name="filename"
default filename
-----------------------------7d429871607fe--

所以只要拼一个这样的byte[] data数据Post过去，就可以达到同样的效果了。但是一定要注意，对于这种带有文件上传的，其ContentType是不一样的，例如上面的这种，其ContentType为"multipart/form-data; boundary=---------------------------7d429871607fe"。有了ContentType，我们就可以知道boundary(就是上面的"---------------------------7d429871607fe")，知道boundary了我们就可以构造出我们所需要的byte[] data了，最后，不要忘记，把我们构造的ContentType传到WebClient中（例如:webClient.Headers.Add("Content-Type", ContentType);）这样，就可以通过WebClient.UploadData 方法上载文件数据了。

具体代码如下：
生成二进制数据类的封装
using System;
using System.Web;
using System.IO;
using System.Net;
using System.Text;
using System.Collections;

namespace UploadData.Common
...{
    /**//// <summary>
    /// 创建WebClient.UploadData方法所需二进制数组
    /// </summary>
    public class CreateBytes
    ...{
        Encoding encoding = Encoding.UTF8;

        /**//// <summary>
        /// 拼接所有的二进制数组为一个数组
        /// </summary>
        /// <param name="byteArrays">数组</param>
        /// <returns></returns>
        /// <remarks>加上结束边界</remarks>
        public byte[] JoinBytes(ArrayList byteArrays)
        ...{
            int length = 0;
            int readLength = 0;

            // 加上结束边界
            string endBoundary = Boundary + "--\r\n"; //结束边界
            byte[] endBoundaryBytes = encoding.GetBytes(endBoundary);
            byteArrays.Add(endBoundaryBytes);

            foreach(byte[] b in byteArrays)
            ...{
                length += b.Length;
            }
            byte[] bytes = new byte[length];

            // 遍历复制
            //
            foreach(byte[] b in byteArrays)
            ...{
                b.CopyTo(bytes, readLength);
                readLength += b.Length;
            }

            return bytes;
        }

        public bool UploadData(string uploadUrl, byte[] bytes, out byte[] responseBytes)
        ...{
            WebClient webClient = new WebClient();
            webClient.Headers.Add("Content-Type", ContentType);

            try
            ...{
                responseBytes = webClient.UploadData(uploadUrl, bytes);
                return true;
            }
            catch (WebException ex)
            ...{
                Stream resp = ex.Response.GetResponseStream();
                responseBytes = new byte[ex.Response.ContentLength];
                resp.Read(responseBytes, 0, responseBytes.Length);               
            }
            return false;
        }

        /**//// <summary>
        /// 获取普通表单区域二进制数组
        /// </summary>
        /// <param name="fieldName">表单名</param>
        /// <param name="fieldValue">表单值</param>
        /// <returns></returns>
        /// <remarks>
        /// -----------------------------7d52ee27210a3c\r\nContent-Disposition: form-data; name=\"表单名\"\r\n\r\n表单值\r\n
        /// </remarks>
        public byte[] CreateFieldData(string fieldName, string fieldValue)
        ...{
            string textTemplate = Boundary + "\r\nContent-Disposition: form-data; name=\"{0}\"\r\n\r\n{1}\r\n";
            string text = String.Format(textTemplate, fieldName, fieldValue);
            byte[] bytes = encoding.GetBytes(text);
            return bytes;
        }

       
        /**//// <summary>
        /// 获取文件上传表单区域二进制数组
        /// </summary>
        /// <param name="fieldName">表单名</param>
        /// <param name="filename">文件名</param>
        /// <param name="contentType">文件类型</param>
        /// <param name="contentLength">文件长度</param>
        /// <param name="stream">文件流</param>
        /// <returns>二进制数组</returns>
        public byte[] CreateFieldData(string fieldName, string filename,string contentType, byte[] fileBytes)
        ...{
            string end = "\r\n";
            string textTemplate = Boundary + "\r\nContent-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n";
           
            // 头数据
            string data = String.Format(textTemplate, fieldName, filename, contentType);
            byte[] bytes = encoding.GetBytes(data);

            // 尾数据
            byte[] endBytes = encoding.GetBytes(end);

            // 合成后的数组
            byte[] fieldData = new byte[bytes.Length + fileBytes.Length + endBytes.Length];

            bytes.CopyTo(fieldData, 0); // 头数据
            fileBytes.CopyTo(fieldData, bytes.Length); // 文件的二进制数据
            endBytes.CopyTo(fieldData, bytes.Length + fileBytes.Length); // \r\n

            return fieldData;
        }

        属性#region 属性
        public string Boundary
        ...{
            get
            ...{
                string[] bArray, ctArray;
                string contentType = ContentType;
                ctArray = contentType.Split(';');
                if (ctArray[0].Trim().ToLower() == "multipart/form-data")
                ...{
                    bArray = ctArray[1].Split('=');
                    return "--" + bArray[1];
                }
                return null;
            }
        }

        public string ContentType
        ...{
            get ...{
                if (HttpContext.Current == null)
                ...{
                    return "multipart/form-data; boundary=---------------------------7d5b915500cee";
                }
                return HttpContext.Current.Request.ContentType;
            }
        }
        #endregion
    }
}

在Winform中调用

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

using UploadData.Common;
using System.IO;

namespace UploadDataWin
...{
    /**//// <summary>
    /// frmUpload 的摘要说明。
    /// </summary>
    public class frmUpload : System.Windows.Forms.Form
    ...{
        private System.Windows.Forms.Label lblAmigoToken;
        private System.Windows.Forms.TextBox txtAmigoToken;
        private System.Windows.Forms.Label lblFilename;
        private System.Windows.Forms.TextBox txtFilename;
        private System.Windows.Forms.Button btnBrowse;
        private System.Windows.Forms.TextBox txtFileData;
        private System.Windows.Forms.Label lblFileData;
        private System.Windows.Forms.Button btnUpload;
        private System.Windows.Forms.OpenFileDialog openFileDialog1;
        private System.Windows.Forms.TextBox txtResponse;
        /**//// <summary>
        /// 必需的设计器变量。
        /// </summary>
        private System.ComponentModel.Container components = null;

        public frmUpload()
        ...{
            //
            // Windows 窗体设计器支持所必需的
            //
            InitializeComponent();

            //
            // TODO: 在 InitializeComponent 调用后添加任何构造函数代码
            //
        }

        /**//// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        protected override void Dispose( bool disposing )
        ...{
            if( disposing )
            ...{
                if (components != null)
                ...{
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        Windows 窗体设计器生成的代码#region Windows 窗体设计器生成的代码
        /**//// <summary>
        /// 设计器支持所需的方法 - 不要使用代码编辑器修改
        /// 此方法的内容。
        /// </summary>
        private void InitializeComponent()
        ...{
            this.lblAmigoToken = new System.Windows.Forms.Label();
            this.txtAmigoToken = new System.Windows.Forms.TextBox();
            this.lblFilename = new System.Windows.Forms.Label();
            this.txtFilename = new System.Windows.Forms.TextBox();
            this.btnBrowse = new System.Windows.Forms.Button();
            this.txtFileData = new System.Windows.Forms.TextBox();
            this.lblFileData = new System.Windows.Forms.Label();
            this.btnUpload = new System.Windows.Forms.Button();
            this.openFileDialog1 = new System.Windows.Forms.OpenFileDialog();
            this.txtResponse = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            //
            // lblAmigoToken
            //
            this.lblAmigoToken.Location = new System.Drawing.Point(40, 48);
            this.lblAmigoToken.Name = "lblAmigoToken";
            this.lblAmigoToken.Size = new System.Drawing.Size(72, 23);
            this.lblAmigoToken.TabIndex = 0;
            this.lblAmigoToken.Text = "AmigoToken";
            //
            // txtAmigoToken
            //
            this.txtAmigoToken.Location = new System.Drawing.Point(120, 48);
            this.txtAmigoToken.Name = "txtAmigoToken";
            this.txtAmigoToken.Size = new System.Drawing.Size(248, 21);
            this.txtAmigoToken.TabIndex = 1;
            this.txtAmigoToken.Text = "";
            //
            // lblFilename
            //
            this.lblFilename.Location = new System.Drawing.Point(40, 96);
            this.lblFilename.Name = "lblFilename";
            this.lblFilename.Size = new System.Drawing.Size(80, 23);
            this.lblFilename.TabIndex = 2;
            this.lblFilename.Text = "Filename";
            //
            // txtFilename
            //
            this.txtFilename.Location = new System.Drawing.Point(120, 96);
            this.txtFilename.Name = "txtFilename";
            this.txtFilename.Size = new System.Drawing.Size(248, 21);
            this.txtFilename.TabIndex = 3;
            this.txtFilename.Text = "";
            //
            // btnBrowse
            //
            this.btnBrowse.Location = new System.Drawing.Point(296, 144);
            this.btnBrowse.Name = "btnBrowse";
            this.btnBrowse.TabIndex = 4;
            this.btnBrowse.Text = "浏览...";
            this.btnBrowse.Click += new System.EventHandler(this.btnBrowse_Click);
            //
            // txtFileData
            //
            this.txtFileData.Location = new System.Drawing.Point(120, 144);
            this.txtFileData.Name = "txtFileData";
            this.txtFileData.Size = new System.Drawing.Size(168, 21);
            this.txtFileData.TabIndex = 5;
            this.txtFileData.Text = "";
            //
            // lblFileData
            //
            this.lblFileData.Location = new System.Drawing.Point(40, 144);
            this.lblFileData.Name = "lblFileData";
            this.lblFileData.Size = new System.Drawing.Size(72, 23);
            this.lblFileData.TabIndex = 6;
            this.lblFileData.Text = "FileData";
            //
            // btnUpload
            //
            this.btnUpload.Location = new System.Drawing.Point(48, 184);
            this.btnUpload.Name = "btnUpload";
            this.btnUpload.TabIndex = 7;
            this.btnUpload.Text = "Upload";
            this.btnUpload.Click += new System.EventHandler(this.btnUpload_Click);
            //
            // txtResponse
            //
            this.txtResponse.Location = new System.Drawing.Point(136, 184);
            this.txtResponse.Multiline = true;
            this.txtResponse.Name = "txtResponse";
            this.txtResponse.Size = new System.Drawing.Size(248, 72);
            this.txtResponse.TabIndex = 8;
            this.txtResponse.Text = "";
            //
            // frmUpload
            //
            this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
            this.ClientSize = new System.Drawing.Size(400, 269);
            this.Controls.Add(this.txtResponse);
            this.Controls.Add(this.btnUpload);
            this.Controls.Add(this.lblFileData);
            this.Controls.Add(this.txtFileData);
            this.Controls.Add(this.btnBrowse);
            this.Controls.Add(this.txtFilename);
            this.Controls.Add(this.lblFilename);
            this.Controls.Add(this.txtAmigoToken);
            this.Controls.Add(this.lblAmigoToken);
            this.Name = "frmUpload";
            this.Text = "frmUpload";
            this.ResumeLayout(false);

        }
        #endregion

        /**//// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        ...{
            Application.Run(new frmUpload());
        }

        private void btnUpload_Click(object sender, System.EventArgs e)
        ...{
            // 非空检验
            if (txtAmigoToken.Text.Trim() == "" || txtFilename.Text == "" || txtFileData.Text.Trim() == "")
            ...{
                MessageBox.Show("Please fill data");
                return;
            }

            // 所要上传的文件路径
            string path = txtFileData.Text.Trim();

            // 检查文件是否存在
            if (!File.Exists(path))
            ...{
                MessageBox.Show("{0} does not exist!", path);
                return;
            }

            // 读文件流
            FileStream fs = new FileStream(path, FileMode.Open,
                FileAccess.Read, FileShare.Read);
           
            // 这部分需要完善
            string ContentType = "application/octet-stream";
            byte[] fileBytes = new byte[fs.Length];
            fs.Read(fileBytes, 0, Convert.ToInt32(fs.Length));

            // 生成需要上传的二进制数组
            CreateBytes cb = new CreateBytes();
            // 所有表单数据
            ArrayList bytesArray = new ArrayList();
            // 普通表单
            bytesArray.Add(cb.CreateFieldData("FileName", txtFilename.Text));
            bytesArray.Add(cb.CreateFieldData("AmigoToken", txtAmigoToken.Text));
            // 文件表单
            bytesArray.Add(cb.CreateFieldData("FileData", path
                                                , ContentType, fileBytes));

            // 合成所有表单并生成二进制数组
            byte[] bytes = cb.JoinBytes(bytesArray);
           
            // 返回的内容
            byte[] responseBytes;
           
            // 上传到指定Url
            bool uploaded = cb.UploadData("http://localhost/UploadData/UploadAvatar.aspx", bytes, out responseBytes);

            // 将返回的内容输出到文件
            using (FileStream file = new FileStream(@"c:\response.text", FileMode.Create, FileAccess.Write, FileShare.Read))
            ...{
                file.Write(responseBytes, 0, responseBytes.Length);
            }

            txtResponse.Text = System.Text.Encoding.UTF8.GetString(responseBytes);

        }

        private void btnBrowse_Click(object sender, System.EventArgs e)
        ...{
            if(openFileDialog1.ShowDialog() == DialogResult.OK)
            ...{
                txtFileData.Text = openFileDialog1.FileName;
            }

        }
    }
}