2次元の List (foreach, LINQ 使用可) ref: http://qiita.com/_meki/items/306cf7c99c04b65431f7

List2D.cs

using System.Linq;

namespace System.Collections.Generic
{
    public class List2D<T> : IEnumerable<List2dIndexer<T>> where T : new()
    {
        public List2D(int w, int h)
        {
            mWidth = w;
            mHeight = h;

            mArr = new List<T>(w * h);

            foreach (int i in Enumerable.Range(1, w * h))
            {
                mArr.Add(new T());
            }
        }

        public T get(int x, int y) { return mArr[x + y * mWidth]; }
        public void set(int x, int y, T val) { mArr[x + y * mWidth] = val; }

        public T this[int x, int y]
        {
            set { mArr[x + y * mWidth] = value; }
            get { return mArr[x + y * mWidth]; }
        }

        public int Count { get { return mWidth * mHeight; } }
        public int Width { get { return mWidth; } }
        public int Height { get { return mHeight; } }

        public List2dIterator<T> GetEnumerator()
        {
            return new List2dIterator<T>(this);
        }

        IEnumerator<List2dIndexer<T>> IEnumerable<List2dIndexer<T>>.GetEnumerator()
        {
            return GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        protected int mWidth;
        protected int mHeight;
        protected List<T> mArr;
    }

    public class List2dIterator<T> : IEnumerator<List2dIndexer<T>> where T : new()
    {
        public List2dIterator(List2D<T> lst)
        {
            indexer = new List2dIndexer<T>();
            indexer.x = -1;
            indexer.y = 0;
            indexer.container = lst;
        }

        public List2dIndexer<T> Current
        {
            get
            {
                return indexer;
            }
        }

        object IEnumerator.Current
        {
            get
            {
                return Current;
            }
        }

        public void Dispose() { }

        public bool MoveNext()
        {
            ++indexer.x;
            if (indexer.x == indexer.container.Width)
            {
                indexer.x = 0;
                indexer.y++;
            }
            return (indexer.y != indexer.container.Height);
        }

        public void Reset()
        {
            indexer.x = -1;
            indexer.y = 0;
        }

        List2dIndexer<T> indexer;
    }

    public class List2dIndexer<T> where T : new()
    {
        public T Value { get { return container[x, y]; } set { container[x, y] = value; } }

        public int X { get { return x; } }
        public int Y { get { return y; } }

        public bool isLastCol { get { return x == container.Width - 1; } }

        internal List2D<T> container;
        internal int x;
        internal int y;
    }
}

main.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class Test
{
    public static void Main()
    {
        // 幅 20, 高さ 20 の2次元 List 作成
        var list = new List2D<int>(20, 20);

        // 中心座標 (10.0, 10.0) から距離 5 以内の点を抽出
        var query = from i in list
                    where (10.0 - i.X) * (10.0 - i.X) + (10.0 - i.Y) * (10.0 - i.Y) <= 25
                    select i;

        // クエリで抽出した点に 1 を代入
        // i の型は List2dIndexer<int>
        foreach(var i in query) { i.Value = 1; }

        // コンソール出力
        foreach(var i in list)
        {
            Console.Write(i.Value == 1 ? "*" : ".");

            if(i.isLastCol)
            {   // 行末で改行
                Console.WriteLine("");
            }
        }
    }
}

コンソール出力

....................
....................
....................
....................
....................
..........*.........
.......*******......
......*********.....
......*********.....
......*********.....
.....***********....
......*********.....
......*********.....
......*********.....
.......*******......
..........*.........
....................
....................
....................
....................