C# exception

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C#3.0

C#

Hi,
I have windows (wordnet api) which is load in button click

frmWordnetAPI relatedWords = new frmWordnetAPI();
relatedWords.ShowDialog();

This API is getting some data from (source file) directory. When compling the program in first button click the form is loading perfectly, but when I close API and re click the button to load it is throwing an exception saying that
The process cannot access the file 'D:\MIT\project\wordnet API\TestApplication\WNdb-3.0\dict\index.adj' because it is being used by another process.

The following code is throwing an exception

BinarySearchTextStream searchStream = new BinarySearchTextStream(indexPath, new BinarySearchTextStream.SearchComparisonDelegate(delegate(string searchWord, string currentLine)
{
    // if we landed on the header text, search further down
    if (currentLine[0] == ' ')
        return 1;
    string currentWord = currentLine.Substring(0, currentLine.IndexOf(' '));
    return searchWord.CompareTo(currentWord);
}));

Please help