namespace TwoComboBoxs
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        NorthwindDataSet.ProductsDataTable productTable = new NorthwindDataSet.ProductsDataTable();
        NorthwindDataSet.CategoriesDataTable categoryTable = new NorthwindDataSet.CategoriesDataTable();
        
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            
            DataRowView rowView = comboBox1.SelectedItem as DataRowView;
         
           

             NorthwindDataSet.CategoriesRow row = rowView.Row as NorthwindDataSet.CategoriesRow;
         
            int id = row.CategoryID;
            productTable.DefaultView.RowFilter = string.Format("CategoryID = {0}", id);
            
        }
      

        private void Form1_Load(object sender, EventArgs e)
        {
            
            NorthwindDataSetTableAdapters.CategoriesTableAdapter categoryAdapter = new TwoComboBoxs.NorthwindDataSetTableAdapters.CategoriesTableAdapter();
            categoryAdapter.Fill(categoryTable);
             = categoryTable.DefaultView;
            comboBox1.DisplayMember = categoryTable.CategoryNameColumn.ColumnName;
            

            NorthwindDataSetTableAdapters.ProductsTableAdapter productAdapter = new NorthwindDataSetTableAdapters.ProductsTableAdapter();
            productAdapter.Fill(productTable);
             = productTable.DefaultView;
            comboBox2.DisplayMember = productTable.ProductNameColumn.ColumnName;
           
           


        }

    }
}
这里面用的是northwind,看不太懂，有没有sql server 相关的语句可以替换一下的

千万别沉呀！！！ 有没有神呀！！！

觉得这里面comboBox1的查询应该和id有关：int id = row.CategoryID;            productTable.DefaultView.RowFilter = string.Format("CategoryID = {0}", id);

comboBox2和ProductsDataTable表有关，但是不太清楚这两表 ProductsDataTable和 CategoriesDataTable之间有什么联系！！         

sql查询的话需要知道查询什么，现在对这两个表只知道CategoriesDataTable有一个id，当然你可以查询所有的项目： select * from 加上表名

仅供参考

那换个问题吧
  DataSet ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter("select G_number from 商品表", conn);
            da.Fill(ds, "G_number");
            DataTable dt = ds.Tables[0];
            comboBox2.DataSource = dt;
            comboBox2.DisplayMember = "G_number";
            comboBox2.SelectedIndex = -1;    是form_load 中对combobox2的出定义，连接商品表中的G_number

  DataSet ds1 = new DataSet();
            SqlDataAdapter da1 = new SqlDataAdapter("select G_name from 商品表", conn);
            DataTable dt1 = ds.Tables[0];
            comboBox3.DataSource = dt1;
            comboBox3.DisplayMember = "G_name";
            comboBox3.ValueMember = "G_name";
            comboBox3.SelectedIndex = -1;  是form_load 中对combobox3的出定义，连接商品表中的G_name
 想选定combobox2中的下拉项时，combobox3中的值跟着改变


 private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            string strSQL1 = "Select G_name from 商品表 where G_number=";
            strSQL1 += comboBox2.Text;
            DataSet ds1 = new DataSet();
            SqlDataAdapter da1 = new SqlDataAdapter(strSQL1, conn);
            da1.Fill(ds1, "商品表");
            DataTable dt1 = ds1.Tables["商品表"];
            comboBox3.DataSource = dt1;
            comboBox3.DisplayMember = "G_name";
            comboBox3.SelectedIndex = -1;

            

        }

现在提示错误无法绑定由多个部分组成的标识符 "System.Data.DataRowView"。
为什么

你的这两句string strSQL1 = "Select G_name from 商品表 where G_number=";            strSQL1 += comboBox2.Text;运行可以通过吗？
从逻辑上讲应该是Select G_name from 商品表 where G_number='comboBox2.Text所显示的值';

所以建议：string strSQL1 = "Select G_name from 商品表 where G_number='";            strSQL1 =strSQL1+ comboBox2.Text+"'";

有点乱

那句没有问题可以通过

可以选到需要的数据吗？

按照LZ的意思 俺试着仿做了一下 没出现那个问题
有可能是这样
你绑定数据有问题，
要不就是你绑定数据的查询语句有问题

俺以前遇到过类似的问题 但是数据源是泛型 貌似咱们的情况不同 如果不好用 建议LZ把代码打包后在上传 这样大家看着都方便