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Comparing two lists and deleting identical results c#

I have two lists:
List<int> positionsThatCannotBeMovedTo =...
List<int> desiredLocations =...
I am trying to remove all of the positions which cannot be moved to from the desired locations to create a list of safe positions:
List<int> safePositions = new List<int>(uniquePositions);
safePositions.RemoveAll(positionsThatCannotBeMovedTo);
however it's throwing the error:
"Argument1: cannot convert from 'System.Collections.Generic.List' to 'System.Predicate'
I'm not entirely sure what this means or how I'm misusing the function. Is anybody able to explain this for me please? I am doing it this way because of the answer in this question:
Compare two lists for updates, deletions and additions

RemoveAll takes a Predicate<T>, but you are passing a list:
safePositions.RemoveAll(x => positionsThatCannotBeMovedTo.Contains(x));

There is another way to obtain a list with elements except the elements of another list
List<int> positionsThatCannotBeMovedTo = new List<int>() {1,2,3,4,5,6,7};
List<int> uniquePositions = new List<int>() {5,6,7,8,9,10};
List<int> safePosition = uniquePositions.Except(positionsThatCannotBeMovedTo).ToList();
MSDN on Enumerable<T>.Except

You could also accomplish this using the Except extension method. Assuming uniquePositions is your list of all your positions.
var safePositions = uniquePositions.Except(positionsThatCannotBeMovedTo).ToList();
Except is the set difference operator and as you are using lists of ints the default comparer is fine.

Is there a Linq method to 'extend' collection?

I wonder if there is a Linq method to simply enumerate over another collection after the current one? If not, why not?
To highlight exactly what I mean, imagine we had:
List<int> a = new List<int> { 1, 2, 3 };
int[] b = { 1, 5, 6 };
then I am asking if there is a Linq method such that a.MyHypotheticalExtensionMethod(b) would produce the IEnumerable containing: {1,2,3,1,5,6}
Of course, its trivial to roll one's own (or even to just work around) but it definately seems like something that ought to be included in Linq?

You're looking for a.Concat(b).

copying one Array value to another array

I have 2 array of object. 1st array of object have property which I want to copy to other array.
1st array of object
HotelRoomResponse[] hr=new HotelRoomResponse[100];
2nd array of object
RateInfos[] rt = new RateInfos[100];
now what i want to do is copy a property of 1st array like
rt=hr[].RateInfo;
but it give error. What is correct way to do this????

You can't just project an array like that. You effectively have to loop - although you don't need to do that manually in your own code. LINQ makes it very easy, for example:
RateInfos[] rt = hr.Select(x => x.RateInfo).ToArray();
Or you could use Array.ConvertAll:
RateInfos[] rt = Array.ConvertAll(hr, x => x.RateInfo);
In both of these cases there's still a loop somewhere - it's just not in your code.
If you're quite new to C# and don't understand LINQ, lambda expressions, delegates etc yet, then you could just write the code yourself:
RateInfos[] rt = new RateInfos[hr.Length];
for (int i = 0; i < rt.Length; i++)
{
rt[i] = hr[i].RateInfo;
}
All of these three will achieve the same result.
The first approach is probably the most idiomatic in modern C#. It will work with any input type, and you can change from ToArray() to ToList() to get a List<RateInfos> instead of an array, etc.
The second approach is slightly more efficient than the first and will work with .NET 2.0 (whereas LINQ was introduced in .NET 3.5) - you'll still need a C# 3 compiler or higher though. It will only work as written with arrays, but there's a similar ConvertAll method for List<T>.
The third approach is the most efficient, but obviously more code as well. It's simpler for a newcomer to understand, but doesn't express what you're trying to achieve as clearly when you know how all the language features work for the first two solutions.

RateInfos[] rt = hr.Select(item => item.RateInfo).ToArray();

Use LINQ:
RateInfos[] rt = hr.Select(x => x.RateInfo).ToArray();

C# assigning elements of a list to a list of variables

In Perl one can do the following
($a, $b, $c) = split(',', "aaa,bbb,ccc");
does anyone know if there is an equivalent in C# other than doing the following?
var elements = "aaa,bbb,ccc".Split(',');
var a = elements[0];
var b = elements[1];
var c = elements[2];
Or is there an alternative for doing the above more concisely?

No. There's no way of assigning more than one variable in a single assignment expression in C#. Do you definitely need separate variables instead of an array?
Perhaps if you gave us the wider context, we may be able to suggest a better approach to the overall problem - often if you try to approach a task in the way that you would in a different language, you end up with messy code, and that may be the case here.

No there is no other way to do this in C#.
But there is hope in .net - namely F# :D
With this you could do
let [| a; b; c |] = "aaa,bbb,ccc".Split(',')

Still not a perfect solution but with C# 7 we can use tuple deconstruction:
var elements = "aaa,bbb,ccc".Split(',');
var (a, b, c) = (elements[0], elements[1], elements[2]);

Easiest way to display array of values in table in windows form C#

I have an array(rows) of arrays which contains 4 elements which I would like to display in a grid or table in a Windows form in C#.
What is the most straightforward way to accomplish this? Is there a way to bind a data grid to an array (or object perhaps)?
What is your recommendation?

You're looking for the DataGridView control.
Instead of an array of arrays, you should make a class with four properties, then make a BindingList<T> of that class.

as SLaks said, that is a more structured way of doing it,
here is a way that uses Linq + Anonymous Types to do something similar on the fly:
int[] numbers = new int[] { 1, 2, 3, 4, 5 };
var evens = from n in numbers select new { Digit = n, Even = (n % 2 == 0) } ;
dataGridView1.DataSource = evens.ToList();
Also if you are after nice ways of displaying then have a look at WPF beats Windows Form hands down.