SpriteSheet and AnimatedSpriteSheet classes for Monogame.

AnimatedSpriteSheet.cs

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Graphics;
using System;

namespace AdvancedWars
{
    class AnimatedSpriteSheet
    {
        private string baseName;
        private int frameCount;
        private int tileWidth;
        private int tileHeight;
        private int tilePadding;

        private SpriteSheet[] sheets;

        public AnimatedSpriteSheet(string baseName, int frameCount, int tileWidth, int tileHeight, int tilePadding = 0)
        {
            this.baseName = baseName;
            this.frameCount = frameCount;
            this.tileWidth = tileWidth;
            this.tileHeight = tileHeight;
            this.tilePadding = tilePadding;

            sheets = new SpriteSheet[frameCount];
        }

        public void LoadContent(ContentManager content)
        {
            for (int i = 0; i < frameCount; i++)
                sheets[i] = new SpriteSheet(content.Load<Texture2D>(baseName + i.ToString()), tileWidth, tileHeight, tilePadding);
        }

        public void DrawSprite(SpriteBatch batch, int spriteIndex, Vector2 location, int frame)
        {
            if (frame < 0 || frame >= frameCount)
                throw new IndexOutOfRangeException("Tried to draw a frame that's either less than 0, or greater than the number of frame in the sheet.");

            sheets[frame].DrawSprite(batch, spriteIndex, location);
        }

        public void DrawSpriteSet(SpriteBatch batch, int[] spriteSet, int rowLength, Vector2 location, int frame)
        {
            if (frame < 0 || frame >= frameCount)
                throw new IndexOutOfRangeException("Tried to draw a frame that's either less than 0, or greater than the number of frame in the sheet.");

            sheets[frame].DrawSpriteSet(batch, spriteSet, rowLength, location);
        }
    }
}

SpriteSheet.cs

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace AdvancedWars
{
    class SpriteSheet
    {
        Texture2D sheet;
        int tileWidth;
        int tileHeight;
        int padding;

        int sheetWidthInTiles;
        int sheetHeightInTiles;

        public int TileCount { get; private set; }

        Rectangle[] SourceRects;

        public SpriteSheet(Texture2D sheet, int tileWidth, int tileHeight, int padding = 0)
        {
            this.sheet = sheet;
            this.tileHeight = tileHeight;
            this.tileWidth = tileWidth;
            this.padding = padding;
            CalculateSheet();
        }

        private void CalculateSheet()
        {
            sheetWidthInTiles = sheet.Width / (tileWidth + padding) + (padding > 0 ? 1 : 0);
            sheetHeightInTiles = sheet.Height / (tileHeight + padding) + (padding > 0 ? 1 : 0);
            TileCount = sheetWidthInTiles * sheetHeightInTiles;
            SourceRects = new Rectangle[TileCount];

            int index = 0;
            for (int y = 0; y < sheetHeightInTiles; y++)
                for (int x = 0; x < sheetWidthInTiles; x++)
                    SourceRects[index++] = new Rectangle((x * tileWidth) + (x * padding), (y * tileHeight) + (y * padding), tileWidth, tileHeight);
        }

        public void DrawSprite(SpriteBatch batch, int spriteIndex, Vector2 location)
        {
            batch.Draw(sheet, location, SourceRects[spriteIndex], Color.White);
        }

        public void DrawSpriteSet(SpriteBatch batch, int[] spriteSet, int rowLength, Vector2 location)
        {
            int x = 0;
            int y = 0;
            for (int i = 0; i < spriteSet.Length; i++)
            {
                if (spriteSet[i] != -1)
                    batch.Draw(sheet, location + new Vector2(tileWidth * x, tileHeight * y), SourceRects[spriteSet[i]], Color.White);
                x++;
                if (i % rowLength == rowLength - 1)
                {
                    y++;
                    x = 0;
                }
            }
        }
    }
}