how to deserialize a json string in c#

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hi to all..
if any body knows how to desrialize a json string...
i have some scenario like this:
i have json string like this:
string strjson = "{"\"firstname\":\"hello\",\"lastname\":\"hai\"}";
how to deserialize tat string in c# pls let me know its very urgent for me...
if any knows pls reply immediately...
and do the need full.......