轉換失敗而轉換爲varchar到int背後

Question

代碼：

protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e) 
{ 
    string countryid = Convert.ToString(DropDownList3.SelectedValue); 
    con.Open(); 

    SqlCommand cmd = new SqlCommand("select * from states where countryID=" + countryid, con); 

    SqlDataAdapter da = new SqlDataAdapter(cmd); 
    DataSet ds = new DataSet(); 
    da.Fill(ds); 

    con.Close(); 

    DropDownList4.DataSource = ds; 
    DropDownList4.DataTextField = "stateName"; 
    DropDownList4.DataValueField = "stateID"; 
    DropDownList4.DataBind(); 

    DropDownList4.Items.Insert(0,new ListItem("--select--","0")); 

    if (DropDownList4.SelectedValue == "0") 
    { 
     DropDownList6.Items.Clear(); 
     DropDownList6.Items.Insert(0, new ListItem("--select--", "0")); 
    } 

表結構：

CREATE TABLE countries 
(
    countryID varchar(3) NOT NULL, 
    countryName nvarchar(100) NOT NULL, 
    localName nvarchar(100), 
    webCode varchar(2), 
    region varchar(50), 
    continent varchar(25), 
    latitude float NOT NULL, 
    longitude float NOT NULL, 
    surfaceArea float NOT NULL, 
    population int NOT NULL, 

    PRIMARY KEY (countryID), 
    UNIQUE (webCode), 
    UNIQUE (countryName) 
); 

錯誤：

System.Data.SqlClient.SqlException: Conversion failed when converting the varchar value 'USA' to data type int.

Answer 1

使用這一個：

SqlCommand cmd = new SqlCommand("select * from states where countryID='" + countryid + "'", con); 

其實你錯過了單引號和你的表CountryID是varchar所以你必須使用單引號