You have to escape the “‘s if you use the @ symbol it doesn’t allow the \ to be used as an escape after the first “. So the two options are:
don’t use the @ and use \ to escape the ”
string someJson = "{\"ErrorMessage\": \"\",\"ErrorDetails\": {\"ErrorID\": 111,\"Description\":{\"Short\": 0,\"Verbose\": 20},\"ErrorDate\": \"\"}}";
or use double quotes
string someJson =@"{""ErrorMessage"": """",""ErrorDetails"": {""ErrorID"": 111,""Description"": {""Short"": 0,""Verbose"": 20},""ErrorDate"": """"}}";