Update all objects in a collection using LINQ

While you can use a ForEach extension method, if you want to use just the framework you can do

collection.Select(c => {c.PropertyToSet = value; return c;}).ToList();

The ToList is needed in order to evaluate the select immediately due to lazy evaluation.

I actually found an extension method that will do what I want nicely

public static IEnumerable<T> ForEach<T>(
    this IEnumerable<T> source,
    Action<T> act)
{
    foreach (T element in source) act(element);
    return source;
}

Although you specifically asked for a LINQ solution and this question is quite old I post a non-LINQ-solution. This is because LINQ (= language integrated query) is meant to be used for queries on collections. All LINQ-methods don’t modify the underlying collection, they just return a new one (or more precise an iterator to a new collection). Thus whatever you do e.g. with a Select doesn’t effect the underlying collection, you simply get a new one.

Of course you could do it with a ForEach (which isn't LINQ, by the way, but an extension on List<T>). But this literally uses foreach anyway, but with a lambda-expression. Apart from this every LINQ method internally iterates your collection e.g. by using foreach or for, however it simply hides it from the client. I don’t consider this any more readable nor maintainable (think of edit your code while debugging a method containing lambda-expressions).

Having said this shouldn't use LINQ to modify items in your collection. A better way is the solution you already provided in your question. With a classic loop you can easily iterate your collection and update its items. In fact all those solutions relying on List.ForEach are nothing different, but far harder to read from my perspective.

So you shouldn't use LINQ in those cases where you want to update the elements of your collection.

There is no built-in extension method to do this. Although defining one is fairly straight forward. At the bottom of the post is a method I defined called Iterate. It can be used like so

collection.Iterate(c => { c.PropertyToSet = value;} );

Iterate Source

public static void Iterate<T>(this IEnumerable<T> enumerable, Action<T> callback)
{
    if (enumerable == null)
    {
        throw new ArgumentNullException("enumerable");
    }

    IterateHelper(enumerable, (x, i) => callback(x));
}

public static void Iterate<T>(this IEnumerable<T> enumerable, Action<T,int> callback)
{
    if (enumerable == null)
    {
        throw new ArgumentNullException("enumerable");
    }

    IterateHelper(enumerable, callback);
}

private static void IterateHelper<T>(this IEnumerable<T> enumerable, Action<T,int> callback)
{
    int count = 0;
    foreach (var cur in enumerable)
    {
        callback(cur, count);
        count++;
    }
}

No, LINQ doesn't support a manner of mass updating. The only shorter way would be to use a ForEach extension method - Why there is no ForEach extension method on IEnumerable?

My 2 pennies:-

 collection.Count(v => (v.PropertyToUpdate = newValue) == null);

You can use LINQ to convert your collection to an array and then invoke Array.ForEach():

Array.ForEach(MyCollection.ToArray(), item=>item.DoSomeStuff());

Obviously this will not work with collections of structs or inbuilt types like integers or strings.

Some people consider this is a comment, but for me is an answer, because the right way to do something wrong is not do it. So, the answer for this question is in the question itself.

DO NOT USE LINQ to modify data. Use a loop.

You can use Magiq, a batch operation framework for LINQ.

Here is the extension method I use...

    /// <summary>
    /// Executes an Update statement block on all elements in an  IEnumerable of T
    /// sequence.
    /// </summary>
    /// <typeparam name="TSource">The source element type.</typeparam>
    /// <param name="source">The source sequence.</param>
    /// <param name="action">The action method to execute for each element.</param>
    /// <returns>The number of records affected.</returns>
    public static int Update<TSource>(this IEnumerable<TSource> source, Func<TSource> action)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (action == null) throw new ArgumentNullException("action");
        if (typeof (TSource).IsValueType)
            throw new NotSupportedException("value type elements are not supported by update.");

        var count = 0;
        foreach (var element in source)
        {
            action(element);
            count++;
        }
        return count;
    }

I assume you want to change values inside a query so you could write a function for it

void DoStuff()
{
    Func<string, Foo, bool> test = (y, x) => { x.Bar = y; return true; };
    List<Foo> mylist = new List<Foo>();
    var v = from x in mylist
            where test("value", x)
            select x;
}

class Foo
{
    string Bar { get; set; }
}

But not shure if this is what you mean.

Quoting Adi Lester's answer (https://stackoverflow.com/a/5755487/8917485)

I quite like this answer, but this answer has a bug. It just changes values in a new created list. It must be changed to two lines to read the real changed list.

var aList = collection.ToList();
aList.ForEach(c => c.PropertyToSet = value);

Suppose we have data like below,

var items = new List<string>({"123", "456", "789"});
// Like 123 value get updated to 123ABC ..

and if we want to modify the list and replace the existing values of the list to modified values, then first create a new empty list, then loop through data list by invoking modifying method on each list item,

var modifiedItemsList = new List<string>();

items.ForEach(i => {
  var modifiedValue = ModifyingMethod(i);
  modifiedItemsList.Add(items.AsEnumerable().Where(w => w == i).Select(x => modifiedValue).ToList().FirstOrDefault()?.ToString()) 
});
// assign back the modified list
items = modifiedItemsList;