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Create expression <> for filtering by custom property

I want to write filter controls that take the object type T

and property name and return Expression<Func<T, bool>>

that checks the value of the passed property. I don't want to use reflection because I am afraid that expressions like this cannot be used by EF. I cannot use delegates because C # has no delegates for properties. What can I do? Maybe I should use a different approach for writing these controls?

Here's my first approach using reflection:

public string FilteringField { get; set; }
public Expression<Func<T, bool>> GetFilterExpression()
{
  if (cmbValue.SelectedIndex == 1)
    return (o => (bool)typeof(T).GetProperty(FilteringField).GetValue(o, null));
  if (cmbValue.SelectedIndex == 2)
    return (o => !(bool)typeof(T).GetProperty(FilteringField).GetValue(o, null));
  return null;
}
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2 answers


Reflection is not a problem here; EF won't even be able to tell the difference. By the way, the delegate approach is not a starter (since you mention EF); ultimately, it is something like:

public static IQueryable<T> Where<T>(this IQueryable<T> query,
    string propertyName, object value)
{
    PropertyInfo prop = typeof(T).GetProperty(propertyName);
    var param = Expression.Parameter(typeof(T), "x");
    var body = Expression.Equal(
        Expression.Property(param, prop),
        Expression.Constant(value, prop.PropertyType)
        );
    var predicate = Expression.Lambda<Func<T, bool>>(body, param);
    return query.Where(predicate);
}


Note that with help Expression.PropertyOrField(propertyName)

you can make this easier; the reason i didn't use is that it's very convenient to know the member type ( prop.PropertyType

) here when creating a constant - otherwise you might get problems with nulls.

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I know this is an old answer, but in case anyone sees this, I have built this project:

https://github.com/PoweredSoft/DynamicLinq

Which should be downloaded and on nuget:



https://www.nuget.org/packages/PoweredSoft.DynamicLinq

and you could just do

query.Where("FirstName", ConditionOperators.Equal, "David");
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