实体框架DbSet动态查询

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.

Answer 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.

Answer 2

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; }