How to allow duplication of LINQ to Entity queries when queries differ only in property?

Question

I have two DbSets, Foo and Bar. Foo has an identification string property, FooName and Bar has an identification string property, BarName.

I am developing a very simple search function where a user request can be either equal or contained in an identifying name.

So, I have two methods (greatly simplified):

public ActionView SearchFoo(string query) 
{
    var equalsQuery = db.Foo.Where(f => f.FooName.Equals(query));
    var containsQuery = db.Foo.Where(f => f.FooName.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}


public ActionView SearchBar(string query) 
{
    var equalsQuery = db.Bar.Where(f => f.BarName.Equals(query));
    var containsQuery = db.Bar.Where(f => f.BarName.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}

Obviously I need a helper method:

public IList<T> Search<T>(string query, DbSet<T> set) 
{
    var equalsQuery = set.Where(f => ???.Equals(query));
    var containsQuery = set.Where(f => ???.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}

At first I tried to add a parameter Func<T, string>to the search parameters, where I could use f => f.FooNameit b => b.BarNameaccordingly, but LINQ to Entities does not support the lambda expression during query execution.

I scratch my head on how I can extract this duplication.

Answer 1

Expression<Funt<T,string>>

public IList<T> Search<T>(string query, DbSet<T> set, Expression<Func<T, string>> propExp)
{
    MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
    ConstantExpression someValue = Expression.Constant(query, typeof(string));
    MethodCallExpression containsMethodExp = 
             Expression.Call(propExp.Body, method, someValue);
    var e = (Expression<Func<T, bool>>)
              Expression.Lambda(containsMethodExp, propExp.Parameters.ToArray());
    var containsQuery = set.Where(e).Take(10);

    BinaryExpression equalExpression = Expression.Equal(propExp.Body, someValue);
    e = (Expression<Func<T, bool>>)
                 Expression.Lambda(equalExpression, propExp.Parameters.ToArray());

    var equalsQuery =  set.Where(e);

    var result = equalsQuery.Union(containsQuery).ToList();
}

:

Search ("myValue", fooSet, foo=>foo.FooName);

, :

public static IList<T> Search<T>(this DbSet<T> set, 
                                  string query, Expression<Func<T, string>> propExp)

:

FooSet.Search ("myValue", foo=>foo.FooName);

Answer 2

:

public interface IName
{ 
    string Name { get; set; }
} 

IName .

 public class Bar : IName { ... }
 public class Foo : IName { ... }

:

public IList<T> SearchByName<T>(string query, DbSet<T> set) 
      where T: class, IName
{
    var equalsQuery = set.Where(f => f.Name.Equals(query));
    var containsQuery = set.Where(f => f.Name.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}

Answer 3

ToString()

public class foo
{
    public string FooName
    {
        get;
        set;
    }

    public override string ToString()
    {
        return FooName;
    }
}

public class Bar
{
    public string BarName
    {
        get;
        set;
    }

    public override string ToString()
    {
        return BarName;
    }
}

public IList<T> Search<T>(string query, DbSet<T> set)
{
    var equalsQuery = set.AsEnumerable().Where(f => f.ToString().Equals(query));
    var containsQuery = set.AsEnumerable().Where(f => f.ToString().Contains(query)).Take(10); 
    var result = equalsQuery.Union(containsQuery).ToList(); . . . // go on to return a view
}

Answer 4

. Expression :

private Expression<Func<T, bool>> GetExpression<T>(string propertyName, string propertyValue, string operatorName)
{
    var parameterExp = Expression.Parameter(typeof(T));
    var propertyExp = Expression.Property(parameterExp, propertyName);
    MethodInfo method = typeof(string).GetMethod(operatorName, new[] { typeof(string) });
    var someValue = Expression.Constant(propertyValue, typeof(string));
    var methodExp = Expression.Call(propertyExp, method, someValue);

    return Expression.Lambda<Func<T, bool>>(methodExp, parameterExp);
}

So you can use this method, there propertyNamewill be FooName and BarName :

public IList<T> Search<T>(string propertyName, string query, DbSet<T> set) 
{
    var equalsQuery = set.Where(GetExpression<T>(propertyName, query, "Equals"));
    var containsQuery = set.Where(GetExpression<T>(propertyName, query, "Contains")).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    return result;
}