SqlCommand Parameter error : "The variable name has already been declared"

Question

我想在单击“提交”按钮后逐行插入所有项目，但不知何故我的变量“@DrinkName”相同并且出现错误。请查看我的代码和屏幕截图以便更好地理解。

Category.aspx.cs

protected void BtnSubmit_Click(object sender, EventArgs e)
{
        string connString = "sql database";
        string insertCommand = "INSERT INTO tbDrinks ( DrinkName, DateOfOrder, Qty, UserName, UserCompany) " +
           "values(@DrinkName, @DateOfOrder, @Qty, @UserName, @UserCompany)"; //for testing purpose

        using (SqlConnection conn = new SqlConnection(connString))
        {
            conn.Open();

            using (SqlCommand sqlcmd = new SqlCommand(insertCommand, conn))
            {                         
                    sqlcmd.Parameters.AddWithValue("@DrinkName", lblCoffee.Text);
                    sqlcmd.Parameters.AddWithValue("@DrinkName", lblEnglishTea.Text);
                    sqlcmd.Parameters.AddWithValue("@DateOfOrder", DateTime.Today);
                    sqlcmd.Parameters.AddWithValue("@Qty", ddlCoffee.SelectedValue);
                    sqlcmd.Parameters.AddWithValue("@Qty", ddlEnglishTea.SelectedValue);
                    sqlcmd.Parameters.AddWithValue("@UserName", txtUserName.Text);
                    sqlcmd.Parameters.AddWithValue("@UserCompany", txtCompanyName.Text);

                    sqlcmd.ExecuteNonQuery();
            }              
        }
}

这是我创建的 Category.aspx，每个项目都有不同的下拉列表。

<table  class="auto-style6">
    <tr>
        <td class="auto-style9">
            <strong>
                <asp:Label ID="lblCompany" runat="server" Text="Company Name : "></asp:Label>
            </strong>
        </td>
        <td class="auto-style10">
            <strong>
                <asp:TextBox ID="txtCompanyName" runat="server"></asp:TextBox>
            </strong>
        </td>           
    </tr>
</table> 

 <table id="tbHotDrinks" class="auto-style3" border="1" bordercolor="#1FC3F3" runat="server">

    <tr>
        <td>
            <asp:Label ID="lblCoffee" runat="server" Text="Coffee"></asp:Label>
        </td>
        <td class="auto-style2">
            <asp:Image ID="coffee" runat="server" Height="76px" ImageUrl="~/images/coffee.gif" Width="99px" ImageAlign="TextTop" />
        </td>
        <td class="auto-style11">
            <asp:DropDownList ID="ddlCoffee" runat="server">
                <asp:ListItem>Qty</asp:ListItem>
                <asp:ListItem>1</asp:ListItem>
                <asp:ListItem>2</asp:ListItem>
                <asp:ListItem>3</asp:ListItem>
                <asp:ListItem>4</asp:ListItem>
                <asp:ListItem>5</asp:ListItem>
                <asp:ListItem>6</asp:ListItem>
                <asp:ListItem>7</asp:ListItem>
                <asp:ListItem>8</asp:ListItem>
                <asp:ListItem>9</asp:ListItem>
            </asp:DropDownList>
        </td>

    </tr>
    <tr>
        <td>
            <asp:Label ID="lblEnglishTea" runat="server" Text="English Tea"></asp:Label>
        </td>
        <td class="auto-style2">
            <asp:Image ID="Image1" runat="server" Height="76px" ImageUrl="~/images/EnglishTea.gif" Width="99px" ImageAlign="TextTop" />
        </td>
        <td class="auto-style11">
            <asp:DropDownList ID="ddlEnglishTea" runat="server">
                <asp:ListItem>Qty</asp:ListItem>
                <asp:ListItem>1</asp:ListItem>
                <asp:ListItem>2</asp:ListItem>
                <asp:ListItem>3</asp:ListItem>
                <asp:ListItem>4</asp:ListItem>
                <asp:ListItem>5</asp:ListItem>
                <asp:ListItem>6</asp:ListItem>
                <asp:ListItem>7</asp:ListItem>
                <asp:ListItem>8</asp:ListItem>
                <asp:ListItem>9</asp:ListItem>
            </asp:DropDownList>
        </td>

    </tr>
</table>

<asp:Button ID="BtnSubmit" runat="server" OnClick="BtnSubmit_Click" Text="Submit" />

下面是我想逐行添加的示例数据库屏幕截图。

我在Category.aspx.cs中尝试过的代码如下。不知何故，它仍然不正确。我只想要最简单的方法。

    protected void BtnSubmit_Click(object sender, EventArgs e)
    {
        string connString = "Data Source";

        string insertCommand = "INSERT INTO tbDrinks ( DrinkName, DateOfOrder, Qty, UserName, UserCompany) " +
                               "VALUES (@DrinkName, @DateOfOrder, @Qty, @UserName, @UserCompany)";

        using (SqlConnection conn = new SqlConnection(connString))
        {
            conn.Open();

            using (SqlCommand sqlcmd = new SqlCommand(insertCommand, conn))
            {                         
                    sqlcmd.Parameters.AddWithValue("@DrinkName", lblCoffee.Text);
                  //  sqlcmd.Parameters.AddWithValue("@DrinkName", lblEnglishTea.Text);
                    sqlcmd.Parameters.AddWithValue("@DateOfOrder", DateTime.Today);
                    sqlcmd.Parameters.AddWithValue("@Qty", ddlCoffee.SelectedValue);
                  //  sqlcmd.Parameters.AddWithValue("@Qty", ddlEnglishTea.SelectedValue);
                    sqlcmd.Parameters.AddWithValue("@UserName", txtUserName.Text);
                    sqlcmd.Parameters.AddWithValue("@UserCompany", txtCompanyName.Text);

                    sqlcmd.ExecuteNonQuery();
            }

            using (SqlCommand sqlcmd1 = new SqlCommand(insertCommand, conn))
            {
                // sqlcmd.Parameters.AddWithValue("@DrinkName", lblCoffee.Text);
                sqlcmd1.Parameters.AddWithValue("@DrinkName", lblEnglishTea.Text);
                sqlcmd1.Parameters.AddWithValue("@DateOfOrder", DateTime.Today);
                // sqlcmd.Parameters.AddWithValue("@Qty", ddlCoffee.SelectedValue);
                sqlcmd1.Parameters.AddWithValue("@Qty", ddlEnglishTea.SelectedValue);
                sqlcmd1.Parameters.AddWithValue("@UserName", txtUserName.Text);
                sqlcmd1.Parameters.AddWithValue("@UserCompany", txtCompanyName.Text);

                sqlcmd1.ExecuteNonQuery();
            }
        }
    }

Answer 1

首先，您要添加两次相同的参数。这需要通过更新该特定参数的值来纠正。

其次，无需创建两个 SqlCommand 实例，您只需更新第二次插入的参数值，然后运行 ExecuteNonQuery() 第二次使用更新后的值。你可以这样做：

 protected void BtnSubmit_Click (object sender, EventArgs e) {
     string connString = "Data Source";
     string insertCommand = "INSERT INTO tbDrinks ( DrinkName, DateOfOrder, Qty, UserName, UserCompany) " +
         "values(@DrinkName, @DateOfOrder, @Qty, @UserName, @UserCompany)";

     using (SqlConnection conn = new SqlConnection (connString)) {
         conn.Open();
         using (SqlCommand sqlcmd = new SqlCommand (insertCommand, conn)) {

             sqlcmd.Parameters.AddWithValue("@DrinkName", lblCoffee.Text);
             sqlcmd.Parameters.AddWithValue("@DateOfOrder", DateTime.Today);
             sqlcmd.Parameters.AddWithValue("@Qty", ddlCoffee.SelectedValue);
             sqlcmd.Parameters.AddWithValue("@UserName", txtUserName.Text);
             sqlcmd.Parameters.AddWithValue("@UserCompany", txtCompanyName.Text);
             sqlcmd.ExecuteNonQuery();

             // update the values and insert with updated values.   
             sqlcmd.Parameters["@DrinkName"].Value = lblEnglishTea.Text; 
             sqlcmd.Parameters["@Qty"].Value = ddlEnglishTea.SelectedValue;
             sqlcmd.ExecuteNonQuery();

         }
     }
 }

Answer 2

你有错误，因为你用相同的名称声明了两次 Sql 参数：

sqlcmd.Parameters.AddWithValue("@Qty", ddlCoffee.SelectedValue);
sqlcmd.Parameters.AddWithValue("@DrinkName", lblEnglishTea.Text);

尝试如下：

protected void BtnSubmit_Click(object sender, EventArgs e) 
{
 string connString = "sql database";
 string insertCommand = "INSERT INTO tbDrinks ( DrinkName, DateOfOrder, Qty, UserName, UserCompany) " +
  "values(@DrinkName, @DateOfOrder, @Qty, @UserName, @UserCompany)"; //for testing purpose

 using(SqlConnection conn = new SqlConnection(connString)) 
 {
  conn.Open();
  using(SqlCommand sqlcmd = new SqlCommand(insertCommand, conn)) {

   sqlcmd.Parameters.AddWithValue("@DrinkName", lblCoffee.Text);
   // sqlcmd.Parameters.AddWithValue("@DrinkName", lblEnglishTea.Text);
   sqlcmd.Parameters.AddWithValue("@DateOfOrder", DateTime.Today);
   sqlcmd.Parameters.AddWithValue("@Qty", ddlCoffee.SelectedValue);
   // sqlcmd.Parameters.AddWithValue("@Qty", ddlEnglishTea.SelectedValue);
   sqlcmd.Parameters.AddWithValue("@UserName", txtUserName.Text);
   sqlcmd.Parameters.AddWithValue("@UserCompany", txtCompanyName.Text);
   sqlcmd.ExecuteNonQuery();
  }
 }
}

Answer 3

如果您想简化代码以保存两次不同的购买并使其更易于维护，
创建一个 class，它将根据给定的参数插入一条记录，并重复使用此 class 来添加多行。
C# 是面向对象的语言，因此我们可以从中受益。

// Represents data which can be different for different drinks
public class DrinkSelection
{
    public string Name { get; set; }
    public int Qty { get; set; }
}

public class DrinkPurchase
{
    private readonly string _userName;
    private readonly string _userCompany;

    public DrinkPurchase(string userName, string userCompany)
    {
        _userName = userName;
        _userCompany = userCompany;
    }

    public void Save(DrinkSelection drink)
    {
        var insert = @"
            INSERT INTO tbDrinks (DrinkName, DateOfOrder, Qty, UserName, UserCompany)
            VALUES (@DrinkName, @DateOfOrder, @Qty, @UserName, @UserCompany)
        ";

        var parameters = new[]
        {
          new SqlParameter("@DrinkName", SqlDbType.Varchar) { Value = drink.Name },
          new SqlParameter("@DateOfOrder", SqlDbType.DateTime) { Value = DateTime.Today },
          new SqlParameter("@Qty", SqlDbType.Varchar) { Value = drink.Qty },
          new SqlParameter("@UserName", SqlDbType.Varchar) { Value = _userName },
          new SqlParameter("@UserCompany", SqlDbType.Varchar) { Value = _userCompany }
        };

        using (var connection = new SqlConnection(connectionString)) 
        using (var command = connection.CreateCommand())
        {
            command.CommandText = insert;
            command.Parameters.AddRange(parameters);

            connection.Open();
            command.ExecuteNonQuery();
        }
    }
}

用法

protected void BtnSubmit_Click(object sender, EventArgs e)
{
    var purchase = new DrinkPurchase(txtUserName.Text, txtCompanyName.Text);

    var coffee = new DrinkSelection 
    { 
        Name = lblCoffee.Text, 
        Qty = ddlCoffee.SelectedValue 
    };
    var tea = new DrinkSelection 
    { 
        Name = lblEnglishTea.Text, 
        Qty = ddlEnglishTea.SelectedValue 
    };

    purchase.Save(coffee);
    purchase.Save(tea);
}

通过将逻辑提取到专用 class，您可以从对 Web 框架的依赖中提取业务逻辑。

SqlCommand Parameter error : "The variable name has already been declared"

SqlCommand Parameter error : "The variable name has already been declared"

c#

sqlclient