In my c# EF Core 6 application I am looking for a solution to dynamically choose the fetched columns in the SELECT statement. I found an interesting approach here enter link description here that allows to dynamically generate an object[] array with the columns to be fetched. Based on the linked question, the code looks for instance like this:
List<Expression> initializers = new List<Expression>();
var topicExprParam = Expression.Parameter(typeof(Topic), "topic");
if (addComp1)
{
Expression<Func<Topic, object>> comp1Expr = topic => topic.Comp1;
initializers.Add(new ParameterSubstituter(topicExprParam).Visit(comp1Expr.Body));
}
if (addComp2)
{
Expression<Func<Topic, object>> comp2Expr = topic => topic.Comp2;
initializers.Add(new ParameterSubstituter(topicExprParam).Visit(comp2Expr.Body));
}
if (addComp3)
{
Expression<Func<Topic, object>> comp3Expr = topic => topic.Comp3;
initializers.Add(new ParameterSubstituter(topicExprParam).Visit(comp3Expr.Body));
}
// eventually some more elements ...
NewArrayExpression newArrExpr = Expression.NewArrayInit(typeof(object), initializers);
Expression<Func<Topic, object[]>> lambda = Expression.Lambda<Func<Topic, object[]>>(newArrayExpr, topicExprParam);
using (var dbContext = _contextFactory.CreateDbContext())
{
var myDynamicList = await dbContext.Topic
.Where(x => ...) // some filters
.Select(lambda)
.ToListAsync<dynamic>();
// not dynamic, hard-coded selected properties and object-array indexes ....
var myTopicRecordsList = myDynamicList.Select(x => new Topic { Comp1 = x[0], Comp2 = x[1], Comp3 = x[2] }).ToList();
}
The problem of the above code is that the result list needs hard-coded selected properties and object-array indexes. Maybe somebody finds the possibility to have also this part generated dynamically. Alternatively instead of generating a dynamic expression corresponding to this:
Query.Select(topic => new object[] { topic.Comp1, topic.Comp2, topic.Comp3 });
I am looking for a preferred solution generating a dynamic expression corresponding to this:
Query.Select(topic => new TopicDTO { TheComp1 = topic.Comp1, TheComp2 = topic.Comp2, TheComp3 = topic.Comp3 });
where the DTO class looks like this:
public class TopicDTO
{
public int TheComp1 { get; set; }
public int TheComp2 { get; set; }
public int TheComp3 { get; set; }
public int TheComp4 { get; set; }
public int TheComp5 { get; set; }
// and so on
}
Does anyone know how to adapt the above code to construct dynamically a LINQ expression corresponding to this query based on a DTO class? The idea is to decide dynamically which DTO elements should be filled from the corresponding database table columns. The solution should not be based on strings containing the names of the columns
