Question

string companyName="ABC";
var query = from q in context.Company where q.CompanyName == companyName select q;

有没有办法用字符串变量替换查询的q.CompanyName部分那么用于过滤的字段是参数？

我试过

string str1 = "companySize";
string str2 = "q." + str1; 
string companySize = "Mid";
var query = from q in context.Company where str2 == companySize select q;

没用。试图让用户选择查询列。

Answer 1

在Dynamic query with Linq

了解以下两个选项的详情

你可以使用其中一个

使用动态LINQ库

以下博客的示例

string strWhere = string.Empty;
string strOrderBy = string.Empty;

if (!string.IsNullOrEmpty(txtAddress.Text))
    strWhere = "Address.StartsWith(\"" + txtAddress.Text + "\")"; 
if (!string.IsNullOrEmpty(txtEmpId.Text))
{
    if(!string.IsNullOrEmpty(strWhere ))
        strWhere = " And ";
    strWhere = "Id = " + txtEmpId.Text;
}
if (!string.IsNullOrEmpty(txtDesc.Text))
{
    if (!string.IsNullOrEmpty(strWhere))
        strWhere = " And ";
    strWhere = "Desc.StartsWith(\"" + txtDesc.Text + "\")";
}
if (!string.IsNullOrEmpty(txtName.Text))
{
    if (!string.IsNullOrEmpty(strWhere))
        strWhere = " And ";
    strWhere = "Name.StartsWith(\"" + txtName.Text + "\")";
}

EmployeeDataContext edb = new EmployeeDataContext();
var emp = edb.Employees.Where(strWhere);

Predicate Builder

示例

    var predicate = PredicateBuilder.True<employee>();

    if(!string.IsNullOrEmpty(txtAddress.Text))
        predicate = predicate.And(e1 => e1.Address.Contains(txtAddress.Text));
    if (!string.IsNullOrEmpty(txtEmpId.Text))
        predicate = predicate.And(e1 => e1.Id == Convert.ToInt32(txtEmpId.Text));
    if (!string.IsNullOrEmpty(txtDesc.Text))
        predicate = predicate.And(e1 => e1.Desc.Contains(txtDesc.Text));
    if (!string.IsNullOrEmpty(txtName.Text))
        predicate = predicate.And(e1 => e1.Name.Contains(txtName.Text));

    EmployeeDataContext edb= new EmployeeDataContext();
    var emp = edb.Employees.Where(predicate);

Answer 2

如果你不想使用像dynamicLINQ这样的库，你可以自己创建表达式树：

string str1 = "companySize";
string str2 = "q." + str1; 
string companySize = "Mid";

var param = Expression.Parameter(typeof(string));
var exp = Expression.Lambda<Func<Company, bool>>(
              Expression.Equal(
                  Expression.Property(param, str1),
                  Expression.Constant(companySize)),
              param);

var query = context.Company.Where(exp);

Answer 3

我认为最好的方法是使用内置库（和PropertyDescriptor类型）。

using System.ComponentModel;

void Main()
{
   Test test = new Test();
   test.CompanyName = "ABC";

   object z = TypeDescriptor.GetProperties(test).OfType<PropertyDescriptor>()
                  .Where(x => x.Name == "CompanyName").Select(x => x.GetValue(test)).FirstOrDefault();

   Console.WriteLine(z.ToString());
}

public class Test
{
   public string CompanyName { get; set; }
}

如何在LINQ中用变量替换变量？

3 个答案: