如何检查字符串字符是否为空格？

我刚刚开始用C＃编程。我正在尝试构建一个简单的Vigenere文本加密工具作为个人项目。

我的问题应该很容易解决，但这确实使我感到压力，无法找到错误。在我的代码中，我试图做一个简单的检查以查看字符串中的字符是否为空格；我已经正确设置了if语句，但是即使第一个测试为true，它也会跳过第一个测试，而转移到else语句。我真的很喜欢这方面的帮助。

我的问题区域在底部。

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

public class fun2013
{
public static void Main()
{
    Console.WriteLine("fun 2013");
    string UserName;
    do
    {
        Console.Write("LOGIN//: ");
        UserName = Console.ReadLine();
    }
    while(UserName != "Max");
    Console.WriteLine(("Hello ") + (UserName) + (", enter your key below."));

                //USER ENTERS TEXT AT THIS POINT

    string loweredPass = Console.ReadLine().ToLower();
        Console.WriteLine("Changing CASE...");
        Console.WriteLine(loweredPass);
    string Strippedpass = loweredPass.Replace(" ","");
        Console.WriteLine("STRIPPING SPACES...");
        Console.WriteLine(Strippedpass);
    int passLength = Strippedpass.Length;
        Console.WriteLine("Enter your message below.");
    string userMessage = Console.ReadLine();
    int MessageLength = userMessage.Length;

                //BEGIN PROCESSING STRINGS INTO ARRAYS

    string temp = "";
    StringBuilder bcon = new StringBuilder();
    char [] passArray = Strippedpass.ToCharArray();
    char [] messArray = userMessage.ToCharArray();
    string letterf = "";

    for(int i=0, j=0; j < (MessageLength); i++, j++)    //i used for key array, j used for message length
        {
        >>> if (messArray[i].ToString() == " ")
            {
                letterf = " ";
            }
        else if (messArray[i].ToString() != " ")
            {
                letterf = passArray[i].ToString();
            }
            if (i >= (passLength-1))    //array starts at value 0, length check starts at 1. Subtract 1 to keep them equal
                {i = -1;}   //-1 is used so it will go back to value of 0 on next loop
        temp = letterf;
        bcon.Append(temp);
        }

    Console.WriteLine();
    Console.WriteLine(bcon);


    Console.WriteLine("Press ENTER to continue...");
        Console.ReadLine(); //KILL APPLICATION
}
}

感谢大家的帮助，但经过进一步检查，我发现我的for循环出错。我正在使用与键数组（int i）相同的时间间隔重置消息数组阅读器。我将其更改为使用正确的整数“ j”。我还将“临时”字符串更新程序和字符串生成器放入循环中的每个if语句中。现在可以正常运行了。

    for (int i=0, j=0; j < (MessageLength); i++, j++)    //i used for key array, j used for message length
    {

    if (messArray[j].ToString() != " ")
        {
            letterf = passArray[i].ToString();
            temp = letterf;
            bcon.Append(temp);
        }

    else if (messArray[j].ToString() == " ")
        {
            letterf = " ";
            temp = letterf;
            bcon.Append(temp);
        }

    if (i >= (passLength-1))    //array starts at value 0, length check starts at 1. Subtract 1 to keep them equal
        {i = -1;}   //-1 is used so it will go back to value of 0 on next loop
    }