Why is my update not persisting?

I have an anonymous type (grps) populated from a linq query. One of the field (DYs) contains an array.

if I run this code:

grps.ElementAt(0).Product = "kkk";

I get a compile error.

If I run this code I get no errors but the value is unchanged.

grps.ElementAt(0).DYs[0] = 19;
Console.WriteLine(grps.ElementAt(0).DYs[0]); // not 19

If, however, I do a foreach on grps and then do a nested step through of each array, I can change the values of the array and they are reported, within the nested loop, as changed. Outside the nested loop they are still unchanged.

I need to make changes to the values in the arrays in my anonymous type but I cannot figure out how.

This has annoyed and confused me because I spent time writing 开发者_如何学运维code that I thought was working ok, but turns out not to without producing any errors.

UPDATE

The upshot to this is that as usual when I have encountered a problem it's because I've forgotten to stick ToList() on the end of something.

Yes, you would get a compile-error for the first one - you're trying to set a property in an anonymous type, and you can't do that in C#. In the second case, you're not trying to set a property - you're just mutating an array. That's an entirely different operation - it's like doing something like:

private readonly StringBuilder builder = new StringBuilder("hello");
...
builder.Append("Stuff");

This changes the contents of the object which builder refers to; it doesn't change the value of the builder variable. Anonymous type properties are read-only in the sense that you can't change their values - but if the value is a reference to a mutable object, you can still mutate the object.

Now, in the second form: you're calling ElementAt twice. That means it will execute the query again the second time - creating a new instance, and thus a new array. The object whose array you changed earlier has gone. However, if you do this:

var list = grps.ToList();
list[0].DYs[0] = 19;
Console.WriteLine(list[0].DYs[0]);

it will print out 19.

Why is my update not persisting?

Unless you used something like .ToList() on your linq query, you are generating a different sequence with each call to ElementAt(0).

The definition of grps is key here... If it's linq... that's your problem.

why does one compile, but the other not compile?

You are asking for an array .DYs, and then you are changing part of the array (then throwing the array away.). That's why the second statement compiles, but the first does not.