Entity Framework DbSet dynamic query

Question

I am creating a DbSet from a Type that is passed in and I need to query the database for a dynamic field and value. Using generics I would use the where function with a lambda expression. Can this be achieved from a standard dbSet created as follows ?

DbSet table = dataContext.Set(EntityType);
PropertyInfo propertyInfo = EntityType.GetProperty(PropertyName);

// Expression: "entity"
ParameterExpression parameter = Expression.Parameter(EntityType, "entity");

// Expression: "entity.PropertyName"
MemberExpression propertyValue = Expression.MakeMemberAccess(parameter, propertyInfo);

// Expression: "value"
object convertedValue = Convert.ChangeType(value, propertyInfo.PropertyType);
ConstantExpression rhs = Expression.Constant(convertedValue);

// Expression: "entity.PropertyName == value"
BinaryExpression equal = Expression.Equal(propertyValue, rhs);

// Expression: "entity => entity.PropertyName == value"
LambdaExpression lambda = Expression.Lambda(equal, parameter);

Now need to query the table to get data.

Not sure if this works with EF. But you could try the dynamic linq lib weblogs.asp.net/scottgu/archive/2008/01/07/… — mxmissile
– mxmissile, Commented Apr 28, 2014 at 13:37
What do you want to achive with this? Maybe there is a better solution... — Christoph Fink
– Christoph Fink, Commented Apr 28, 2014 at 13:46
@chrfin - This is a part of validation Attribute, I am passing the attribute name and entity type and need to query — user3581461
– user3581461, Commented Apr 28, 2014 at 14:00
Can you show a demo entity and call? Because this sounds like it could be solved differently... — Christoph Fink
– Christoph Fink, Commented Apr 28, 2014 at 14:06
Can you give us what the parameters you are getting? I assume a type and is it just the name of the property and an object value or is it an Expression<Func<>> — CharlesNRice
– CharlesNRice, Commented Apr 28, 2014 at 15:06

Mikael Guldborg · Accepted Answer · 2014-04-29 19:48:17Z

1

There is a nuget package called 'System.Linq.Dynamic'.

http://dynamiclinq.codeplex.com/

This package allows you to form statements against DbSets using strings, like this:

myContext.MyDbSet.Where("PropertyName == @0", "aValue");

It is possible to do this, with Expressions as you suggest in your question, but this library takes out all the heavy lifting.

I've used this with great success in one of my previous projects.

answered Apr 29, 2014 at 19:48

Mikael Guldborg

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Christoph Fink · Accepted Answer · 2014-04-30 08:52:43Z

0

You can try something like the following:

public class UniqueRecordValidationAttribute : ValidationAttribute
{
    public IValidationAttribute DynamicInstance { get; private set; }

    public UniqueRecordValidationAttribute(Type type, 
        params object[] arguments )
    {
        DynamicInstance = 
            Activator.CreateInstance(type, arguments) as IValidationAttribute;
    }

    public override bool IsValid(object value)
    {
        return DynamicInstance.IsValid(value);
    }
}

public class UniqueRecordValidator<C, E, P> : IValidationAttribute
    where C : DataContext, new() where E : class
{
    Func<E, P, bool> Check { get; set; }

    public UniqueRecordValidator(Func<E, P, bool> check)
    {
        Check = check;
    }

    public bool IsValid(object value)
    {
        DataContext dataContext = new C();
        Table<E> table = dataContext.GetTable<E>();

        return table.Count(i => Check(i as E, (P)value)) == 0;
    }
}

public interface IValidationAttribute
{
    bool IsValid(object value);
}

and

[UniqueRecordValidation(
    typeof(UniqueRecordValidator<AssetTrackingEntities, ATUser_Account, string>), 
    new Func<ATUser_Account, string, bool>((i, p) =>  i.User_Login == p))]
public string User_Name { get; set; }

This would be a completely strongly typed solution, but I am not shure the Func<E, P, bool> inside the Count is supported by EF as I could not test that here at the moment. But for LINQ to objects this code does work.

If this does not work you can at least improve it with generics and dynamic LINQ to the following:

public class UniqueRecordValidator<C, E> : IValidationAttribute
    where C : DataContext, new() where E : class
{
    string PropertyName { get; set; }

    public UniqueRecordValidator(string propertyName)
    {
        PropertyName = propertyName;
    }

    public bool IsValid(object value)
    {
        DataContext dataContext = new C();
        Table<E> table = dataContext.GetTable<E>();

        return table.Count(PropertyName + " = @0", value) == 0;
    }
}

[UniqueRecordValidation(
    typeof(UniqueRecordValidator<AssetTrackingEntities, ATUser_Account>)
    "User_Login")] 
public string User_Login { get; set; }

edited Apr 30, 2014 at 8:52

answered Apr 29, 2014 at 20:45

Christoph Fink

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2 Comments

user3581461 Over a year ago

Thanks for your suggestions, but are you sure that you can use generics with attributes?? Please check this: stackoverflow.com/questions/9788593/…

Christoph Fink Over a year ago

Ok, I did not know that. You can use something like here: whathecode.wordpress.com/2013/10/19/generic-attributes-in-c - use the attribute just as a wrapper around the actual generic check.

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