Adding a Lambda Expression and Entity Framework

Question

If I want to get more columns with an existing lambda tree expression as below, how would I do it? This works with Entity Frameworks and wants it to still work.

Expression<Func<DivisionTeam, DirectorTeamModel>> columns= (d) => new DirectorTeamModel
{
    Id = d.Id,
    TeamId = d.Team.Id
};

if (criteria.Template == ExportTemplate.Import || criteria.Template == ExportTemplate.Default)
{
    // Retrieve additional columns from "columns" expression tree
}

return _divisionTeamsRepository.GetPagedResults(criteria.Page, criteria.PageSize, @where.Expand(), string.Format("{0} {1}", criteria.SortOrder, criteria.SortDirection), columns);

Answer 1

Given the two selector expressions, you must take the bindings from them MemberInitExpressionand create a new expression using all the bindings. But this expression will not work, because it uses two different parameter expressions for one parameter. We must also fix this.

Considering,...

Expression<Func<TSource, TResult>> left = ... // columns
Expression<Func<TSource, TResult>> right = ... // more columns

... take the bindings ...

var leftInit = left.Body as MemberInitExpression;
var rightInit = right.Body as MemberInitExpression;

var bindings = leftInit.Bindings.Concat(rightInit.Bindings);

... create a new expression ...

var result = Expression.Lambda<Func<TSource, TResult>>(
    Expression.MemberInit(Expression.New(typeof(TResult)), bindings), ???);

... , ...

var binder = new ParameterBinder(left.Parameters[0], right.Parameters[0]);
var bindings = binder.Visit(leftInit.Bindings.Concat(rightInit.Bindings));

// now, just use right.Parameters[0] as parameter...

, , :

class ParameterBinder : ExpressionVisitor
{
    readonly ParameterExpression parameter;
    readonly Expression replacement;

    public ParameterBinder(ParameterExpression parameter, Expression replacement)
    {
        this.parameter = parameter;
        this.replacement = replacement;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node == parameter)
            return replacement;

        return base.VisitParameter(node);
    }
}

. , (: ), - :

var merged = columns.Apply(moreColumns);