C# : System.Collections.Generic.IEnumerable' does not contain any definition for 'ToList'

C# : System.Collections.Generic.IEnumerable' does not contain any definition for 'ToList'
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C# : System.Collections.Generic.IEnumerable' does not contain any definition for 'ToList'
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contain System.Collections.Generic.IEnumerable' does definition : not any C# for 'ToList'