using System;
using System.Text;

public class SimpleColumnarTransposition
{

public static int rownumber(ref string Array,ref int Line)//判断数组的行数
{
int leavenumber=Array.Length%Line;
int row1=(int)(Array.Length/Line);
if (leavenumber==0)
{
return (row1-1);
}
else
{
return (row1);
}
}

public static char[][] getchar(ref int ROW,ref string mingwen,ref int Line)//返回类型要有
{
int count=0;//记是mingwen的第几个字符
char[][] ch=new char [ROW][];

for (int row2=0;row2<ROW;row2++)
{
for (int i=0,number=count;i<Line;i++,number++,count++)
{
while (count<mingwen.Length)
{
ch[row2][i]=mingwen[number];
}
}
}
return ch;
}

public static void Main()
{
int row,line;//row是行数，line是列数
string arg;
Console.WriteLine("输入你想输入的明文：");
arg=Console.ReadLine();
string args=arg.Replace(" ","");
Console.WriteLine("输入一个数字定义是几列的数组：");
line=Convert.ToInt32(Console.ReadLine());
row=rownumber(ref args,ref line);
char[][] cha=getchar(ref row,ref args,ref line);

Console.WriteLine("Press the key number:");

string s=Console.ReadLine();
foreach(char number1 in s)
{
int number2=(int)number1;
for (int x=0;x<row;x++)
{
Console.WriteLine(cha[x][number2]);
}
}
}
}

public static char[][] getchar(ref int ROW,ref string mingwen,ref int Line)//返回类型要有
{
int count=0;//记是mingwen的第几个字符
char[][] ch=new char [ROW][];

for (int row2=0;row2<ROW;row2++)
{
for (int i=0,number=count;i<Line;i++,number++,count++)
{
while (count<mingwen.Length)
{
ch[row2][i]=mingwen[number];
}
}
}
return ch;
}

ch是一个二维数组, 你只声明了第一维的长度, 第二维长度没有声明所以为null

晕死
到现在都没人来帮下我回答下
这里是我编译好通过运行成功的代码，昨天晚上刚调试成功，大家多多交流啊
using System;
using System.Text;

public class SimpleColumnarTransposition
{

public static int rownumber(string Array,int Column)//判断数组的行数
{
int leavenumber=Array.Length%Column;
int row1=(int)(Array.Length/Column);
if (leavenumber==0)
{
return (row1);
}
else
{
return (row1+1);
}
}

public static char[][] getchar(int ROW,string mingwen,int Column)//把字符串中的字符分配到不定数组中去
{
int count=0;//记是mingwen的第几个字符
char[][] ch=new char [ROW][];

for (int row2=0;row2<ROW;row2++)
{
if (row2!=ROW-1)
{
ch[row2]=new char[Column];
for (int i=0,number=count;i<Column;i++,number++,count++)
{
while(count<mingwen.Length)
{
ch[row2][i]=mingwen[number];
break;
}
}
}
else
{
Column=mingwen.Length%Column;
ch[row2]=new char[Column];
for (int i=0,number=count;i<Column;i++,number++,count++)
{
while(count<mingwen.Length)
{
ch[row2][i]=mingwen[number];
break;
}
}
}
}
return ch;
}

public static void Main()
{
int row,column,i;//row是行数，column是列数
string arg;
Console.WriteLine("输入你想输入的明文：");
arg=Console.ReadLine();
string args=arg.Replace(" ","");

Console.WriteLine("输入一个数字定义是几列的数组：");

column=Convert.ToInt32(Console.ReadLine());
row=rownumber(args,column);

char [][]cha=new char[row][];
for (i=0;i<row;i++)
{
cha[i]=new char[column];
}
cha=getchar(row,args,column);

Console.WriteLine("输入你想按列输出的顺序，数字为从0开始到{0}结束的数字，每个数字要且仅输一次:",column-1);
int [] keynumber=new int [column];
for (i=0;i<keynumber.Length;i++)
{
keynumber[i]=(Convert.ToInt32((Console.Read()))-48);
}

for (i=0;i<keynumber.Length;i++)
{
column=keynumber[i];
for (int row3=0;row3<row;row3++)
{
if (row3==row-1&&column>(cha[row-1].Length-1))
break;
else
Console.Write(cha[row3][column]);
}
}
Console.WriteLine();
}
}