Instantiating any number of classes at runtime with only the class name known as a string

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C#

I have a desktop application that collects some data and then does some postprocessing to the data. The post processor varies depending on user selection. Each post processer is a class which inherits the IPostProcessor interface which defines the signature of the "CrunchNumbers" method. So far I have 2 post processor classes but I know that more will be added in the future. Currently I have an "if"-"else if" statement that selects the postprocessor based on the class name passed in as a string. Inside the "else if" it calls the appropriate factory which returns the class instance corresponding to the string specified (see code below). The problem is that I want to be able to add new post processor classes without altering the code that instantiates them. So my question is, how can I write code that will intantitate a class at runtime (that inherits a know interface) for a class that won't be written until some time in the future? In other words, I don't want to have to keep coming back and adding more "else if" lines to my code below.

IPostProcessor biasScaleFactorPP;
IPostProcessor AllanVariancePP;
private void LaunchPostProcessor(double[,] twoDArray, string postProcessorName)
{
    if (postProcessorName == BiasScaleFactorString && !closing)
    {
        if (biasScaleFactorPP == null)
        {
            biasScaleFactorPP = PostProcFactory.CreatePostProcessor<BiasScaleFactor>();
            biasScaleFactorPP.Initialize(serialNumbers, dataId, populatedLocations);
        }
        bsfDelegate = new DelegateToMethod(biasScaleFactorPP.CrunchNumbers);
        bsfDelegate(twoDArray, PostProcParams);
    }
    else if (postProcessorName == AllanVarianceString && !closing)
    {
        if (AllanVariancePP == null)
        {
            AllanVariancePP = PostProcFactory.CreatePostProcessor<AllanVariance>();
            AllanVariancePP.Initialize(serialNumbers, dataId, populatedLocations);
        }
        avDelegate = new DelegateToMethod(AllanVariancePP.CrunchNumbers);
        avDelegate(twoDArray, PostProcParams);
    }
    else if (!closing)
    {
        throw new Exception(string.Concat(postProcessorName, " is not recognized as a valid post processor"));
    }
}