How to get Json Property name using reflection in C#

You can use Json.NET's own contract resolver for this purpose. Doing so will correctly handle properties with, and without, [JsonProperty(string name)] attributes added, as well as objects with naming strategies or data contract attributes applied directly.

First add the following method:

public static partial class JsonExtensions
{
    static readonly IContractResolver defaultResolver = JsonSerializer.CreateDefault().ContractResolver;

    public static object GetJsonProperty<T>(T obj, string jsonName, bool exact = false, IContractResolver resolver = null)
    {
        if (obj == null)
            throw new ArgumentNullException(nameof(obj));
        resolver = resolver ?? defaultResolver;
        var contract = resolver.ResolveContract(obj.GetType()) as JsonObjectContract;
        if (contract == null)
            throw new ArgumentException(string.Format("{0} is not serialized as a JSON object", obj));
        var property = contract.Properties.GetProperty(jsonName, exact ? StringComparison.Ordinal : StringComparison.OrdinalIgnoreCase);
        if (property == null)
            throw new ArgumentException(string.Format("Property {0} was not found.", jsonName));
        return property.ValueProvider.GetValue(obj); // Maybe check JsonProperty.Readable first, and throw an exception if not?
    }
}

And now you can do:

var employee = new Employee
{
    LastName = "last name",
};

Console.WriteLine("Value of {0} is {1}.", "emp_lname", 
                  JsonExtensions.GetJsonProperty(employee, "emp_lname")); // Prints Value of emp_lname is last name.

If your app uses camel casing for JSON serialization by default, you can pass a CamelCasePropertyNamesContractResolver in for resolver like so:

Console.WriteLine("Value of {0} is {1}.", "emp_lname", 
                  JsonExtensions.GetJsonProperty(employee, "emp_lname", resolver : new CamelCasePropertyNamesContractResolver())); 

If you need to get a list of all JSON property names for a given type, see Get a list of JSON property names from a class to use in a query string:

public static partial class JsonExtensions
{
    public static string [] PropertyNames(Type type)
    {
        return PropertyNames(defaultResolver, type);
    }

    //Taken from this answer https://stackoverflow.com/a/45829514/3744182
    //To https://stackoverflow.com/questions/33616005/get-a-list-of-json-property-names-from-a-class-to-use-in-a-query-string
    public static string [] PropertyNames(this IContractResolver resolver, Type type)
    {
        if (resolver == null || type == null)
            throw new ArgumentNullException();
        var contract = resolver.ResolveContract(type) as JsonObjectContract;
        if (contract == null)
            return new string[0];
        return contract.Properties.Where(p => !p.Ignored).Select(p => p.PropertyName).ToArray();
    }
}

Demo fiddle here.