Error Can't convrt strng to systm.collectn.generic.dictnry<>

Error Can't convrt string to system.collection.generic.dictionary

code:

HtmlElement paragraph = this.editorWebBrowser.Document.GetElementById(paragraphId);

Dictionary styleDictionary = GetStyleDictionary(paragraph.Style);

string currentMargin = paragraph.Style.ToString();

currentMargin = Regex.Match(currentMargin, @"-?\d+").Value;

int numericMargin = Int32.Parse(currentMargin);

styleDictionary = (numericMargin + 15).ToString() + "px";

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Basically i have to add value to my existing style value by 15;

ie:

Margin-left:15px

then

get that style

and add 15 px more: Margin-left:30px so on...