Question

我有这个代码

// IQueryable<General> query

if (columnName == "Column1") 
{
  query = query.Where(x => x.Column1 == searchValue);
}
else if (columnName == "Column2") 
{
  query = query.Where(x => x.Column2 == searchValue);
}
else if (columnName == "Column3") 
{
  query = query.Where(x => x.Column3 == searchValue);
}
else if (columnName == "Column4") 
{
  query = query.Where(x => x.Column4 == searchValue);
}
// next zilions columns to come
// ...

我的问题是。我怎样才能将x.Column用作“ .where”条件中的参数？

Answer 1

You can create a predicate manually. Use this method:

public static Expression<Func<General, bool>> CreatePredicate(string columnName, object searchValue)
{
    var xType = typeof(General);
    var x = Expression.Parameter(xType, "x");
    var column = xType.GetProperties().FirstOrDefault(p => p.Name == columnName);

    var body = column == null
        ? (Expression) Expression.Constant(true)
        : Expression.Equal(
            Expression.PropertyOrField(x, columnName),
            Expression.Constant(searchValue));

    return Expression.Lambda<Func<General, bool>>(body, x);
}

Now you can apply your predicate:

IQueryable<General> query = //
var predicate = CreatePredicate(columnName , searchValue);
query = query.Where(predicate);

Answer 2

您可以使用Reflection通过名称检索属性

x.GetType().GetProperty(propertyName,BindingFlags).SetValue(x,value)
// propertyName = "Column1" for example
// BindingFlags are most likely Instance, Public and Property (IIRC)

或将PropertyInfo作为参数直接传递到方法中。您的选择取决于要向方法的使用者公开的抽象级别。

Answer 3

You could use reflection and extension methods. As a rough example:

public class Foo
{
  public int Column1 { get; set; }
  public int Column2 { get; set; }
  ...
}

public static class FooExtensions
{
  // I would use the actual type here instead of object if you know the type.
  public static object GetProperyValue(this Foo foo, string columnName)
  {
    var propertyInfo = foo.GetType().GetProperty(columnName);
    var value = propertyInfo.GetValue(foo);
    // as well as cast value to the type
    return value;
  }
}

...

query = query.Where(x => x.GetProperyValue(columnName) == searchValue);
...

As a side note, that is not a well-designed query because every time you add a column to your model, you'd need to update your if-else. It violates the O in SOLID.

LINQ条件与动态列

3 个答案: