Need to build a generic query method using LINQ expressions

Question

I'm new and this is probably very trivial but I have the following method which works fine, using LINQ for MongoDB:

public T Single<T>(System.Linq.Expressions.Expression<Func<T, bool>> whereExpression) where T : class, new()
{
    T retval = default(T);
    using (var db = Mongo.Create(_connectionString))
    {
        retval = db.GetCollection<T>().AsQueryable()
                    .Where(whereExpression).SingleOrDefault();
    }
    return retval;
}

But I would like to add a "Select" to it (for projection) using a parameter as well, something that might look like this (which obviously doesn't work):

public T SingleWithSelect<T>(System.Linq.Expressions.Expression<Func<T, bool>> whereExpression, System.Linq.Expressions.Expression<Func<T, bool>> selectExpression) where T : class, new()
{
    T retval = default(T);
    using (var db = Mongo.Create(_connectionString))
    {
        retval = db.GetCollection<T>().AsQueryable()
                    .Where(whereExpression)
                    .Select(selectExpression)
                    .SingleOrDefault();
    }
    return retval;
}

In the hopes that it'll return something similar to the following:

var results = db.GetCollection<Entity>("Entities").AsQueryable()
.Where(i => i.Id == someId)
.Select(y => new { y.SomeEntity }).SingleOrDefault();

Essentially I just need to know how to pass the SELECT parameter into the return function - surprisingly hard to find the solution online when so unfamiliar with LINQ.

Thank you.

Tim Robinson · Accepted Answer · 2011-01-13 18:22:16Z

2

You need another generic type to represent the result of the Select call.

public TResult SingleWithSelect<T, TResult>(
    Expression<Func<T, bool>> whereExpression,
    Expression<Func<T, TResult>> selectExpression)
    where T : class, new()
{
    TResult retval = default(TResult);
    using (var db = Mongo.Create(_connectionString))
    {
        retval = db.GetCollection<T>().AsQueryable()
                    .Where(whereExpression)
                    .Select(selectExpression)
                    .SingleOrDefault();
    }
    return retval;
}

answered Jan 13, 2011 at 18:22

Tim Robinson

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7 Comments

Mikalee Over a year ago

That's great, thanks Tim! When calling the method using the following I'm getting an error message that I'm trying to implicitly convert Anonymous type to "SomeEntity", do you know how I could remedy that as well? Method call: var result = SingleWithSelect<Entity, SomeEntity>(x => x.Id == id, y => new { y.SomeEntity });

Tim Robinson Over a year ago

What do you want to return - an anonymous type like new { SomeEntity = y.SomeEntity }, or a SomeEntity? The <Entity, SomeEntity> says you want to return a SomeEntity. If you want to return an anonymous type, call var result = SingleWithSelect(x => x.Id ... etc.

Mikalee Over a year ago

I do want to return SomeEntity, I'm just having problems with the syntax (learning LINQ as I go with this)...

Tim Robinson Over a year ago

You probably want: var result = SingleWithSelect<Entity, SomeEntity>(x => x.Id == id, y => y.SomeEntity);. I'd still expect the <...> to be redundant - the compiler can normally figure this part out for itself.

Andrew Orsich Over a year ago

One thing... db.GetCollection<T>().AsQueryable() -> load all collection from mongodb... I think it is not good.

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