SyntaxFix

[c#] Convert IQueryable<> type object to List<T> type?

Here's a couple of extension methods I've jury-rigged together to convert IQueryables and IEnumerables from one type to another (i.e. DTO). It's mainly used to convert from a larger type (i.e. the type of the row in the database that has unneeded fields) to a smaller one.

The positive sides of this approach are:

  • it requires almost no code to use - a simple call to .Transform<DtoType>() is all you need
  • it works just like .Select(s=>new{...}) i.e. when used with IQueryable it produces the optimal SQL code, excluding Type1 fields that DtoType doesn't have.

LinqHelper.cs:

public static IQueryable<TResult> Transform<TResult>(this IQueryable source)
{
    var resultType = typeof(TResult);
    var resultProperties = resultType.GetProperties().Where(p => p.CanWrite);

    ParameterExpression s = Expression.Parameter(source.ElementType, "s");

    var memberBindings =
        resultProperties.Select(p =>
            Expression.Bind(typeof(TResult).GetMember(p.Name)[0], Expression.Property(s, p.Name))).OfType<MemberBinding>();

    Expression memberInit = Expression.MemberInit(
        Expression.New(typeof(TResult)),
        memberBindings
        );

    var memberInitLambda = Expression.Lambda(memberInit, s);

    var typeArgs = new[]
        {
            source.ElementType, 
            memberInit.Type
        };

    var mc = Expression.Call(typeof(Queryable), "Select", typeArgs, source.Expression, memberInitLambda);

    var query = source.Provider.CreateQuery<TResult>(mc);

    return query;
}

public static IEnumerable<TResult> Transform<TResult>(this IEnumerable source)
{
    return source.AsQueryable().Transform<TResult>();
}

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