How to Convert Lambda Expression To Sql?

问题:

I am developing a small framework to access the database. I want to add a feature that makes a query using a lambda expression. How do I do this?

public class TestModel
{
    public int Id {get;set;}
    public string Name {get;set;}
}

public class Repository<T>
{
    // do something.
}

For example:

var repo = new Repository<TestModel>();

var query = repo.AsQueryable().Where(x => x.Name == "test"); 
// This query must be like this:
// SELECT * FROM testmodel WHERE name = 'test'

var list = query.ToDataSet();
// When I call ToDataSet(), it will get the dataset after running the made query.

回答1:

Go on and create a LINQ Provider (I am sure you don't want to do this, anyway).

It's a lot of work, so maybe you just want to use NHibernate or Entity Framework or something like that.

If your queries are rather simple, maybe you don't need a full blown LINQ Provider. Have a look at Expression Trees (which are used by LINQ Providers).

You can hack something like this:

public static class QueryExtensions
{
    public static IEnumerable<TSource> Where<TSource>(this Repo<TSource> source, Expression<Func<TSource, bool>> predicate)
    {
        // hacks all the way
        dynamic operation = predicate.Body;
        dynamic left = operation.Left;
        dynamic right = operation.Right;

        var ops = new Dictionary<ExpressionType, String>();
        ops.Add(ExpressionType.Equal, "=");
        ops.Add(ExpressionType.GreaterThan, ">");
        // add all required operations here            

        // Instead of SELECT *, select all required fields, since you know the type
        var q = String.Format("SELECT * FROM {0} WHERE {1} {2} {3}", typeof(TSource), left.Member.Name, ops[operation.NodeType], right.Value);
        return source.RunQuery(q);
    }
}
public class Repo<T>
{
    internal IEnumerable<T> RunQuery(string query)
    {
        return new List<T>(); // run query here...
    }
}
public class TestModel
{
    public int Id { get; set; }
    public string Name { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var repo = new Repo<TestModel>();
        var result = repo.Where(e => e.Name == "test");
        var result2 = repo.Where(e => e.Id > 200);
    }
}

Please, don't use this as it is. This is just a quick and dirty example how expression trees can be analyzed to create SQL statements.

Why not just use Linq2Sql, NHibernate or EntityFramework...

回答2:

if you want to do things like

db.Employee
.Where(e => e.Title == "Spectre")
.Set(e => e.Title, "Commander")
.Update();

or

db
.Into(db.Employee)
    .Value(e => e.FirstName, "John")
    .Value(e => e.LastName,  "Shepard")
    .Value(e => e.Title,     "Spectre")
    .Value(e => e.HireDate,  () => Sql.CurrentTimestamp)
.Insert();

or

db.Employee
.Where(e => e.Title == "Spectre")
.Delete();

Then check out this, BLToolkit

回答3:

You might want to look at http://iqtoolkit.codeplex.com/ Which is very complex and i dont recommend you to build something from scratch.

I just wrote something close to dkons's answer I will add it anyway. Just using fluent interface nothing more.

public class Query<T> where T : class
{
    private Dictionary<string, string> _dictionary;

    public Query()
    {
        _dictionary = new Dictionary<string, string>();
    } 

    public Query<T> Eq(Expression<Func<T, string>> property)
    {
        AddOperator("Eq", property.Name);
        return this;
    }

    public Query<T> StartsWith(Expression<Func<T, string>> property)
    {
        AddOperator("Sw", property.Name);
        return this;
    }

    public Query<T> Like(Expression<Func<T, string>> property)
    {
        AddOperator("Like", property.Name);
        return this;
    }

    private void AddOperator(string opName, string prop)
    {
        _dictionary.Add(opName,prop);
    }

    public void Run(T t )
    {
        //Extract props of T by reflection and Build query   
    }
}

Lets say you have a model like

class Model
    {
        public string Surname{ get; set; }
        public string Name{ get; set; }
    }

You can use this as :

static void Main(string[] args)
        {

            Model m = new Model() {Name = "n", Surname = "s"};
            var q = new Query<Model>();
            q.Eq(x => x.Name).Like(x=>x.Surname).Run(m);


        }