C# Linq 列名作为变量

Question

我有一个表，我想在其中对变量列进行查询。 喜欢：

private void query(string column, string value) {

    using (var db = new myDB()) {

        var s1 = (from c in db.Components
                  where (**column** == **value**)
                  select new {c.id, **column**});
    }
}

假设我想寻找供应商，那么它会是这样的：

var s1 = (from c in db.Components
          where (c.supplier == "abc")
          select new {c.id, c.supplier});

有没有办法将列名作为变量传递？

Answer 1

我猜这个例子很有用。

 void BindGridTypeSafe()
    {
        NorthwindDataContext northwind = new NorthwindDataContext();

        var query = from p in northwind.Products
                    where p.CategoryID == 3 && p.UnitPrice > 3
                    orderby p.SupplierID
                    select p;

        GridView1.DataSource = query;
        GridView1.DataBind();
    }

    void BindGridDynamic()
    {
        NorthwindDataContext northwind = new NorthwindDataContext();

        var query = northwind.Products
                             .Where("CategoryID = 3 AND UnitPrice > 3")
                             .OrderBy("SupplierID");

        GridView1.DataSource = query;
        GridView1.DataBind();
    }

Answer 2

一个不错的方法是使用Dynamic Linq

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

类似：

var s1 = (from c in db.Components
    where(column + "=" + value)
    select new {c.id, **column**});

Answer 3

简短的回答是添加库 System.Linq.Dynamic 作为参考并执行：

string columnName = "Supplier";
var s1 = Suppliers
    .Where(String.Format("{0} == \"abc\"", columnName))
    .Select(new {c.id, c.supplier};

以下是 Dynamic Linq 的完整工作示例，其中 column-name 是一个参数：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Dynamic;

public class Program
{
    public static void Main()
    {
        var lstContacts = new List<Contact>{
            new Contact{Id = 1, Active = true, Name = "Chris"}, 
            new Contact{Id = 2, Active = true, Name = "Scott"}, 
            new Contact{Id = 3, Active = true, Name = "Mark"}, 
            new Contact{Id = 4, Active = false, Name = "Alan"}};

        string columnName = "Active";
        List<Contact> results = lstContacts.Where(String.Format("{0} == true", columnName)).ToList();

        foreach (var item in results)
        {
            Console.WriteLine(item.Id.ToString() + " - " + item.Name.ToString());
        }
    }
}

public class Contact
{
    public int Id
    {
        get;
        set;
    }

    public bool Active
    {
        get;
        set;
    }

    public string Name
    {
        get;
        set;
    }
}

你可以试试这个.net-fiddle-here

Answer 4

我正在恢复这个旧线程，因为我今天必须使用 ASP.NET Core 2.2 解决这个问题。我使用System.Linq.Dynamic.Core NuGet 包创建了以下扩展方法，如果您需要检查多个给定的字符串值是否包含在多个给定列中，该方法可以很好地工作。

public static IQueryable<TEntity> WhereContains<TEntity>(
    this IQueryable<TEntity> query,
    string field,
    string value,
    bool throwExceptionIfNoProperty = false,
    bool throwExceptionIfNoType = false)
    where TEntity : class
{
    PropertyInfo propertyInfo = typeof(TEntity).GetProperty(field);
    if (propertyInfo != null)
    {
        var typeCode = Type.GetTypeCode(propertyInfo.PropertyType);
        switch (typeCode)
        {
            case TypeCode.String:
                return query.Where(String.Format("{0}.Contains(@0)", field), value);
            case TypeCode.Boolean:
                var boolValue = (value != null
                    && (value == "1" || value.ToLower() == "true"))
                    ? true
                    : false;
                return query.Where(String.Format("{0} == @0", field), boolValue);
            case TypeCode.Int16:
            case TypeCode.Int32:
            case TypeCode.Int64:
            case TypeCode.UInt16:
            case TypeCode.UInt32:
            case TypeCode.UInt64:
                return query.Where(String.Format("{0}.ToString().Contains(@0)", field), value);

            // todo: DateTime, float, double, decimals, and other types.

            default:
                if (throwExceptionIfNoType)
                    throw new NotSupportedException(String.Format("Type '{0}' not supported.", typeCode));
                break;
        }
    }
    else
    {
        if (throwExceptionIfNoProperty)
            throw new NotSupportedException(String.Format("Property '{0}' not found.", propertyInfo.Name));
    }
    return query;
}

该代码可用于 .NETStandard/.NETCore（使用上述System.Linq.Dynamic.Core 包）以及 ASP.NET 4.x（使用System.Linq.Dynamic 包）。

有关 WhereContains 扩展方法的更多信息和完整的用例信息，请查看我博客上的 this post。