How To Write A Proper Linq Query With Join

Can someone help me get this code functioning? I get this error Could not find an implementation of the query pattern for source type ‘orderinfo’. ‘GroupJoin’ not found
And this is my code
Models.PaymentProcessed ps = new PaymentProcesse…

nodos de XML con LINQ

necesito "numFound" y su valor "1", además, necesito todos los "str" ("su atributo y su valor")
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tengo el siguiente código:
XmlDocument xmlDocument = new XmlDocument();