Linq 查询不返回值

本文关键字：返回值 查询 Linq | 更新日期: 2023-09-27 18:30:42

  • 框架：.Net 4.5

我正在用数据库中的数据填充文本框字段。但是，调试后，我发现我的 Linq 查询没有返回值。

protected void btnChoose_Click(object sender, EventArgs e)
{
    API_DatabaseEntities1 db = new API_DatabaseEntities1();
    if (ddlCustomer.SelectedValue == "Marisol") {
        tbDescription.Text = (from c in db.Customers
                                where c.CustomerID == 4
                                select c.ProductDescription).ToString();
        tbFName.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.Fname).ToString();
        tbSocial.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.SSN).ToString();
        tbDOB.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.DOB).ToString();
        tbFName1.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.Fname).ToString();
        tbMName.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.Mname).ToString();
        tbLName.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.Lname).ToString();
        tbPrimaryPhone.Text = (from c in db.Customers
                                where c.CustomerID == 4
                                select c.PrimaryPhone).ToString();
        tbSecondaryPhone.Text = (from c in db.Customers
                                    where c.CustomerID == 4
                                    select c.SecondaryPhone).ToString();
        tbAdd1.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.Address).ToString();
        tbCity.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.City).ToString();
        tbZip.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.Zip).ToString();
        tbEmail.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.Email).ToString();
        tbMonLease.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.MortLeaseAmt).ToString();
        tbEmployer.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.Employer).ToString();
        tbPosition.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.Position).ToString();
        tbHireDate.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.Position).ToString();
        tbWorkPhone.Text = (from c in db.Customers
                            where c.CustomerID == 4
                            select c.WorkPhone).ToString();
        tbGross.Text = (from c in db.Customers
                        where c.CustomerID == 4
                        select c.GrossIncome).ToString();
    }
    Debug.WriteLine(tbGross.Text);
}

此时，我不确定我的数据库连接或 Linq 查询是否存在问题。感谢您提供的任何帮助。

Linq 查询不返回值

我想你的意思是得到一个客户而不是一个列表。

select将返回一个集合而不是一个项目，请像这样更改它：

  var customer = (from c in db.Customers
                 where c.CustomerID == 4
                 select c).FirstOrDefault();

请注意 FirstOrDefault() 的调用，这将返回 1 个客户，而不是包含 1 个客户的列表。

如果查询未返回任何内容，FirstOrDefault()将返回null：

 if(customer == null)
     return; // no customer found

然后，您可以像这样填充文本字段：

   tbDescription.Text = customer.ProductDescription;
   tbFName.Text = customer.Fname;
   etc.

请这样做：

var oneRec = (from c in db.Customer
                  where c.CustomerID == 4
                  select c).FirstOrDefault();
tbDescription.Text = oneRec.ProductDescription;
tbFName.Text = oneRec.Fname;
tbSocial.Text = oneRec.SSN;
// ect
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