如果调用ToList或ToArray，为什么C＃编译器允许在表达式树中使用动态操作

Question

Please note: I am aware that there are many related to the main error message discussed here but I could not find any post that discusses my question.

Therefore, please be kind to read to the end before marking this down as a duplicate.

As always I am happy to take the question down if you could point to a question that discusses the issue I am confused about.

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I have a method that returns a IQueryable like below

public IQueryable<dynamic> GetData(DateTime Start, DateTime End, Nullable<int> EmployeeId = null)
{
    var query = from T1 in Context.Table1
                join T2 in Context.Table2
                on T1.Col1 equals T2.Col1
                join T3 in Context.Table3
                on T2.Col1 equals T3.Col1
                where T1.Col1 == EmployeeId
                        && T1.Col2 >= Start
                        && T1.Col2 <= End
                select new
                {
                    Value1 = T1.Col1,
                    Value2 = T2.Col5,
                    Value3 = T3.Col7

                };

    return query;
}

Please note that I had to remove table and column names due to sensitive nature of the tables.

The problem:

I want to get the sum of Value3 so from a different class.

private float getTargetScore()
{
    return GetData().Sum(rec => Convert.ToSingle(rec.Value3));
}

This gives me below compiler error.

An expression tree may not contain a dynamic operation

The confusing part:

I dont get this compiler error if I add .ToList() or .ToArray() before calling Sum()

private float getTargetScore()
{
    return GetData().ToArray().Sum(rec => Convert.ToSingle(rec.Value3));
}

I understand that .ToList() or .ToArray() materialises the query (brings data from the db in this case) and results in an IEnumerable<> rather than IQueryable<> but why does the C# compiler allow me to use a dynamic operation in an expression here?

Note: Version of C# is 4.0 (.Net 4.0)

Answer 1

In the first case you are calling IQueryable.Sum

public static int Sum<TSource>(this IQueryable<TSource> source,
                                    Expression<Func<TSource, int>> selector)

In the second case you are calling IEnumerable.Sum

 public static int Sum<TSource>(this IEnumerable<TSource> source,
                                     Func<TSource, int> selector)

So in first case compiler tries to create an expression tree from lambda expression and is unable to do it since you are using dynamic.