Selecting Random Item from List given probability of each item

Sorry about badly phrased title....

I have an object called NGram

class NGram
{
     //other properties
     double Probability {get; set;} //Value between 1 and 0 
}

Now suppose I have a list of these objects such that...

List<NGrams> grams = GetNGrams();
Debug.Assert(grams.Sum(x => x.Probability) == 1);

How can I select a random item from this list while factoring in the probability distribution.

For instance, suppose grams[0].Probability == 0.5 then there should be a 50% chance of selecting grams[0]

I figured I may need something like rand.NextDouble() but I am at loss.