Slow crappy implementation of a sparse vector in c#

gistfile1.cs

public double this[string key] { 
  get 
  { 
    if (InnerHashtable.ContainsKey(key)) 
    { 
      return (double)InnerHashtable[key] 
    } 
    else 
    { 
      return 0.0d 
    } 
  } 
  set 
  { 
    InnerHashtable[key] = value; 
  } 
}