public string GenerateBlock(int width, int height)
{
    var sb = new StringBuilder();
    for (int h = 0; h < height; h++)
    {
        var borderRow = h == 0 || h + 1 == height;
        for (int w = 0; w < width; w++)
        {
            var borderColumn = w == 0 || w + 1 == width;
            if (borderRow)
                sb.Append("=");
            else if (borderColumn)
                sb.Append("|");
            else
                sb.Append(".");
        }
        sb.AppendLine();
    }
    return sb.ToString();
}

// Usage:
Console.WriteLine(GenerateBlock(20, 10));

// Result:
====================
|..................|
|..................|
|..................|
|..................|
|..................|
|..................|
|..................|
|..................|
====================