C# 使用LINQ语句在列表中使用Select和Where

这是MySQL表

在使用Visual Studio 2019、C和.NET Framework 4.7开发的ASP.NET网站主页中，我使用MySQL表的值创建了两个列表，如下所示：

List<string> Listp_uni = new List<string>();
List<string> Listp_int = new List<string>();

Container.p_int = reader["p_int"].ToString();
Container.p_uni = reader["p_uni"].ToString();

Listp_uni.Add(Container.p_uni.ToString());
Listp_int.Add(Container.p_int.ToString());

Container.p_uni = string.Join(",", Listp_uni.Select(e => "'" + e + "'").Distinct());
Container.p_int = string.Join(" ", Listp_int.Select(e => "" + e + "").Distinct());

我需要使用LINQ从列表列表p_uni中选择等于X4SVR的值

我尝试过，但没有成功

var studentNames = Mp.Container.p_uni.Where(s => s.Mp.Container.p_uni == "X4SVR")
                   .Select(s => s);

错误是

编译器错误消息：CS1061:“char”不包含定义 对于'Mp'，没有接受第一个参数的扩展方法'Mp' 如果缺少using指令或 汇编参考

我怎么做

编辑问题

新编辑

---

尝试从数据表中执行以下操作：

            DataTable dt = new DataTable();
            dt.Columns.Add("p_int", typeof(int));
            dt.Columns.Add("p_uni", typeof(string));
            
            dt.Rows.Add(new object[] { 0, "seat"});
            dt.Rows.Add(new object[] { 1, "X400"});
            dt.Rows.Add(new object[] { 4, "X400"});
            dt.Rows.Add(new object[] { 2, "X400"});
            dt.Rows.Add(new object[] { 2, "X4SVR"});
            dt.Rows.Add(new object[] { 3, "X400"});


            string p_int = string.Join(",", dt.AsEnumerable().Select(e => e.Field<int>("p_int")).Distinct());
            string p_uni = string.Join(" ", dt.AsEnumerable().Select(e => e.Field<string>("p_uni")).Distinct());

---

首先，您必须了解我们不能在字符串中使用select。 主要功能在链接中，请仔细阅读。 在下面的代码中，我创建了一些用于选择的示例

X4SVR。请查收。我的代码不是一个更好的代码。但是它会给你一些见解来解决你的问题

快乐编码：

        DataTable dt = new DataTable();
        dt.Columns.Add("p_int", typeof(int));
        dt.Columns.Add("p_uni", typeof(string));

        dt.Rows.Add(new object[] { 0, "seat" });
        dt.Rows.Add(new object[] { 1, "X400" });
        dt.Rows.Add(new object[] { 4, "X400" });
        dt.Rows.Add(new object[] { 2, "X400" });
        dt.Rows.Add(new object[] { 2, "X4SVR" });
        dt.Rows.Add(new object[] { 3, "X400" });

        string p_int = string.Join(",", dt.AsEnumerable().Select(x => x.Field<int>("p_int")).Distinct());
        string p_uni = string.Join(" ", dt.AsEnumerable().Select(z => z.Field<string>("p_uni")).Distinct());

        Response.Write(p_int + "<br />" + p_uni + "<br /><br />");

        // if you want to get p_uni and p_int take from the datatable itself
        //it will return matching  p_uni and p_int
        var studentNames = dt.AsEnumerable().Where(s => s.Field<string>("p_uni") == "X4SVR").Select(y => y);

        //if you want get the name only
        //Method - using the array
        string [] p_uniArray = dt.AsEnumerable().Select(z => z.Field<string>("p_uni")).ToArray();
        var studentNames1 = p_uniArray.AsEnumerable().Where(s => s == "X4SVR").Select(y => y);

        //if you need to take from p_uni string it self please convert it into array  like
        //here you are not using any separator so I'm splitting it with space

        var stName = p_uni.Split(" ");
        var studentNames2 = stName.Where(x => x == "X4SVR").Select( y => y).Distinct();

      //  var studentNames = dt.Where(s => s.p_uni == "X4SVR").Select(s => s);

        Response.Write(studentNames);

---

看起来Mp.Container.p_uni包含一个字符串，而不是您可能期望的字符串列表。很可能是'seat'、'X400'、'X4SVR'，…Where中的s参数是字符，而不是您预期的字符串。@gkulshrestha是的Mp.Container.p_uni是'seat'、'X400'、'X4SVR'@gkulshrestha出于好奇，Mp.Container.p_uni是什么？我在这段代码中找不到它：|@AmalPS好的，我用Mp.Container.p_uni编辑了这个问题。一个名为“e”的局部变量不能在这个范围内声明，因为它会赋予“e”不同的含义，它已经在“父或当前”范围内用于表示某些东西else@IterLsicIealf,如果您在此范围内已经有一个变量E，请将其全部说出来。已在“父级或当前”范围中使用。您只需通过本部分中的任何其他内容重命名e即可选择e=>e.@XDT转换好的，请在第一个问题中使用新编辑。我在问题中指出了同样的错误。。。

public static class Container
{
    public static string p_int
    {
        get
        {
            if (HttpContext.Current.Session["p_int"] != null)
            {
                return HttpContext.Current.Session["p_int"].ToString();
            }
            return null;
        }
        set
        {
            HttpContext.Current.Session["p_int"] = value;
        }
    }

    public static string p_uni
    {
        get
        {
            if (HttpContext.Current.Session["p_uni"] != null)
            {
                return HttpContext.Current.Session["p_uni"].ToString();
            }
            return null;
        }
        set
        {
            HttpContext.Current.Session["p_uni"] = value;
        }
    }
}


    string constr = ConfigurationManager.ConnectionStrings["cn"].ConnectionString;

    using (MySqlConnection con =
        new MySqlConnection(constr))
    {
        using (MySqlCommand cmd =
            new MySqlCommand())
        {
            try
            {
                if (username != null)
                {
                    con.Open();
                    cmd.Connection = con;
                    cmd.CommandText = "SP_ML_AUTE";
                    cmd.CommandType = CommandType.StoredProcedure;
                    cmd.Parameters.AddWithValue("tusername", username.ToString().ToUpper());

                    using (MySqlDataReader reader =
                        cmd.ExecuteReader())
                    {
                        if (reader.HasRows)
                        {
                            while (reader.Read())
                            {
                                Container.p_int = reader["p_int"].ToString();
                                Container.p_uni = reader["p_uni"].ToString();
                                Listp_uni.Add(Container.p_uni.ToString());
                                Listp_int.Add(Container.p_int.ToString());
                            }

                            Container.p_uni = string.Join(",", Listp_uni.Select(e => "'" + e + "'").Distinct());
                            Container.p_int = string.Join(" ", Listp_int.Select(e => "" + e + "").Distinct());
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                throw new ApplicationException("operation failed!", ex);
            }
            finally
            {
                cmd.Connection.Close();
            }
        }
    }

DataTable dt = new DataTable();
dt.Columns.Add("p_int", typeof(int));
dt.Columns.Add("p_uni", typeof(string));

dt.Rows.Add(new object[] { 0, "seat" });
dt.Rows.Add(new object[] { 1, "X400" });
dt.Rows.Add(new object[] { 4, "X400" });
dt.Rows.Add(new object[] { 2, "X400" });
dt.Rows.Add(new object[] { 2, "X4SVR" });
dt.Rows.Add(new object[] { 3, "X400" });

string p_int = string.Join(",", dt.AsEnumerable().Select(x => x.Field<int>("p_int")).Distinct());
string p_uni = string.Join(" ", dt.AsEnumerable().Select(z => z.Field<string>("p_uni")).Distinct());

Response.Write(p_int + "<br />" + p_uni + "<br /><br />");

var studentNames = p_uni.Where(s => s.p_uni == "X4SVR").Select(s => s);

Response.Write(studentNames);

            DataTable dt = new DataTable();
            dt.Columns.Add("p_int", typeof(int));
            dt.Columns.Add("p_uni", typeof(string));
            
            dt.Rows.Add(new object[] { 0, "seat"});
            dt.Rows.Add(new object[] { 1, "X400"});
            dt.Rows.Add(new object[] { 4, "X400"});
            dt.Rows.Add(new object[] { 2, "X400"});
            dt.Rows.Add(new object[] { 2, "X4SVR"});
            dt.Rows.Add(new object[] { 3, "X400"});


            string p_int = string.Join(",", dt.AsEnumerable().Select(e => e.Field<int>("p_int")).Distinct());
            string p_uni = string.Join(" ", dt.AsEnumerable().Select(e => e.Field<string>("p_uni")).Distinct());

        DataTable dt = new DataTable();
        dt.Columns.Add("p_int", typeof(int));
        dt.Columns.Add("p_uni", typeof(string));

        dt.Rows.Add(new object[] { 0, "seat" });
        dt.Rows.Add(new object[] { 1, "X400" });
        dt.Rows.Add(new object[] { 4, "X400" });
        dt.Rows.Add(new object[] { 2, "X400" });
        dt.Rows.Add(new object[] { 2, "X4SVR" });
        dt.Rows.Add(new object[] { 3, "X400" });

        string p_int = string.Join(",", dt.AsEnumerable().Select(x => x.Field<int>("p_int")).Distinct());
        string p_uni = string.Join(" ", dt.AsEnumerable().Select(z => z.Field<string>("p_uni")).Distinct());

        Response.Write(p_int + "<br />" + p_uni + "<br /><br />");

        // if you want to get p_uni and p_int take from the datatable itself
        //it will return matching  p_uni and p_int
        var studentNames = dt.AsEnumerable().Where(s => s.Field<string>("p_uni") == "X4SVR").Select(y => y);

        //if you want get the name only
        //Method - using the array
        string [] p_uniArray = dt.AsEnumerable().Select(z => z.Field<string>("p_uni")).ToArray();
        var studentNames1 = p_uniArray.AsEnumerable().Where(s => s == "X4SVR").Select(y => y);

        //if you need to take from p_uni string it self please convert it into array  like
        //here you are not using any separator so I'm splitting it with space

        var stName = p_uni.Split(" ");
        var studentNames2 = stName.Where(x => x == "X4SVR").Select( y => y).Distinct();

      //  var studentNames = dt.Where(s => s.p_uni == "X4SVR").Select(s => s);

        Response.Write(studentNames);