string strData = "This is much easier"
int intDelimiterIndx = strData.IndexOf("is");
int intDelimiterLength = "is".Length;
str1 = strData.Substring(0, intDelimiterIndx);
str2 = strData.Substring(intDelimiterIndx + intDelimiterLength, strData.Length - (intDelimiterIndx + intDelimiterLength));

string s = "This is a sentence.";
string[] res = s.Split(new string[]{ " is " }, StringSplitOptions.None);

for(int i=0; i<res.length; i++)
    Console.Write(res[i]);

编辑:“is”在数组的两边用空格填充，以保持这样一个事实，即你只想从句子中删除“is”一词，而“this”一词保持不变。

下面是一个使用字符串分隔符进行分割的扩展函数:

public static string[] Split(this string value, string seperator)
{
    return value.Split(new string[] { seperator }, StringSplitOptions.None);
}

用法示例:

string mystring = "one[split on me]two[split on me]three[split on me]four";
var splitStrings = mystring.Split("[split on me]");

var dict = File.ReadLines("test.txt")
               .Where(line => !string.IsNullOrWhitespace(line))
               .Select(line => line.Split(new char[] { '=' }, 2, 0))
               .ToDictionary(parts => parts[0], parts => parts[1]);


or 

    enter code here

line="to=xxx@gmail.com=yyy@yahoo.co.in";
string[] tokens = line.Split(new char[] { '=' }, 2, 0);

ans:
tokens[0]=to
token[1]=xxx@gmail.com=yyy@yahoo.co.in

.．.简而言之:

string[] arr = "This is a sentence".Split(new string[] { "is" }, StringSplitOptions.None);

http://msdn.microsoft.com/en-us/library/system.string.split.aspx

文档中的例子:

string source = "[stop]ONE[stop][stop]TWO[stop][stop][stop]THREE[stop][stop]";
string[] stringSeparators = new string[] {"[stop]"};
string[] result;

// ...
result = source.Split(stringSeparators, StringSplitOptions.None);

foreach (string s in result)
{
    Console.Write("'{0}' ", String.IsNullOrEmpty(s) ? "<>" : s);
}