您可以使用简单的Linq扩展名， TakeWhile

You could use simple Linq extensions, SkipWhile and TakeWhile

List<Tuple<string,double>> items = new List<Tuple<string,double>>() 
{ 
    new Tuple<string,double>("q", .5),  
    new Tuple<string,double>("w", 1.5), 
    new Tuple<string,double>("e", .7), 
    new Tuple<string,double>("r", .8), 
    new Tuple<string,double>("q", .5)
};


var sumvalue = items.Sum(c=>c.Item2); // Calculates sum of all values

var betweensum = items.SkipWhile(x=>x.Item1 == "q") // Skip until matching item1            
    .TakeWhile(x=>x.Item1 != "q") // take until matching item1
    .Sum(x=>x.Item2); // Sum

按照评论中的要求，如果您有多个这样的集合，并且想要在多个集合的匹配字符串之间进行计数，请执行此操作.

As asked in the comments, in case if you have multiple such sets and you want count in between those matching strings for multiple sets, do this.

    int gid = 0;
    items.Select(c => new { Tuple = c,  gid = c.Item1=="q"? ++gid : gid })
        .GroupBy(x=>x.gid)
        .Where(x=>x.Key%2==1)
        .SelectMany(x=>x.Skip(1))
        .Sum(x=>x.Tuple.Item2);

工作 Demo