House number parsing
Anytime I encounter a method that's only doing read-only parsing of strings and I see it's using String.IndexOf() and positions, it leaps out at me like a gigantic code smell.

Sometimes upon examination it turns out to be a legitimate and wise usage. However very often the programmer is manually splitting strings and looping through parts of the string. In other words they are doing exactly what String.Split() and foreach loops would offer them, but doing it the longer and less readable way.

This is one of those cases.

This may be just my personal taste in code. In any case, I can show you how I would tackle this problem using foreach and Split(), which I believe would be a great improvement to your methods. Except for this difference I think we might have taken comparable approaches.

Variable names

A lot of your variables are named with very little descriptiveness. You may understand that variables are named so that someone maintaining or reading the code (like me) can understand what's going on, but you could do much better in this department.


result: You use this word many times in your variable names. The problem is this name is completely useless to me. I can see it is the result of something (variously a function, a loop or some other process) but beyond that you might as well just call the variable foo or bar. hauseNummerVon and hauseNummerBin are far more understandable than resultVon and resultBin - I now know exactly what it is; it's the from house number and the to house number.

For either result[] array, I am left with nothing and I would have to read all of the code before I understood what they contained. This is a little backwards - I should know what the variable contains, then be able to read the code - with descriptive variable names giving me a greater understanding of what's going on.

bFound: Don't shorten your variable names like this. I'm left wondering: What is a b? What does it mean if it's found? Maybe this would be more recognisable to a German reader, but even so, expanding b to a word to say exactly what's found would do wonders for readability.
pos1, pos2: SeparatorPosition and SeparatorEnd would have been more descriptive. Better yet these variables are entirely unnecessary if you just use String.Split().


My approach

As I mentioned above, as oppose to editing your methods, I've written from scratch how I'd tackle this problem.

ParseHouseNumberRange

This is the entry method. You feed this method a string such as "20a bis 21c". It splits it up on the "bis" or the "-" (defined in the separators array) in order to produce smaller chunks of the original string.

Each of these chunks is passed to  then passes each of the results (in this case, "20a " and " 21c", spaces included) over to the non-public method ParseHouseNumber below. ParseHouseNumber will split these chunks up further and this method combines all the results into a single array which is returned.

Given "20a bis 21c" or similar, it returns an array: 20, a, 21, c.

public string[] ParseHouseNumberRange(string houseNumberRange)
{
    string[] separators = new string[] {"bis", "-"};
    string[] houseNumbers = houseNumberRange.Split(separators,
        StringSplitOptions.RemoveEmptyEntries);

    List&lt;string&gt; parsedList = new List&lt;string&gt;();

    foreach (string houseNumber in houseNumbers)
    {
        parsedList.AddRange(ParseHouseNumber(houseNumber));
    }

    return parsedList.ToArray();
}


ParseHouseNumber

This method takes a segment of an address from the method above, such as "20a ". After trimming out any spaces, it walks through the string character by character, plucking out digits and letters and sorting them into their own individual sections e.g. "1c4ad3" would be split into the sections 1, c, 4, ad, 3. Any non-digit non-letter characters are ignored.

Once all sections have been sorted out from the string provided to it, it returns those sections in the form of an array.

public string[] ParseHouseNumber(string houseNumber)
{
    bool firstRun = true;
    bool alphabeticMode = false; // set on first run

    houseNumber = houseNumber.Trim();

    List&lt;string&gt; sections = new List&lt;string&gt;();
    string currentSection = "";

    foreach (char c in houseNumber)
    {
        bool isLetter = Char.IsLetter(c);
        bool isDigit = Char.IsDigit(c);

        if (!(isDigit || isLetter)) continue;

        // If we've switched character type, then a section's finished.
        if (firstRun)
        {
            alphabeticMode = isLetter;
            firstRun = false;
        }
        else if ((isLetter &amp;&amp; !alphabeticMode)
            || (isDigit &amp;&amp; alphabeticMode))
        {
            sections.Add(currentSection);
            currentSection = "";
            alphabeticMode = !alphabeticMode;
        }

        currentSection += c;
    }

    if (currentSection != "") sections.Add(currentSection);

    return sections.ToArray();
}