How to input null-value into specflow step definition table

You can create your own IValueRetriever and replace default one with yours

public class StringValueRetriver : IValueRetriever
{
    public bool CanRetrieve(KeyValuePair<string, string> keyValuePair, Type targetType, Type propertyType)
    {
        return propertyType == typeof(string);
    }

    public object Retrieve(KeyValuePair<string, string> keyValuePair, Type targetType, Type propertyType)
    {
        return string.IsNullOrEmpty(keyValuePair.Value) ? null : keyValuePair.Value;
    }
}

Some where in your scenario steps

[BeforeScenario]
public void BeforeScenario()
{
    Service.Instance.ValueRetrievers.Unregister<TechTalk.SpecFlow.Assist.ValueRetrievers.StringValueRetriever>();
    Service.Instance.ValueRetrievers.Register(new StringValueRetriver());
}

older syntax:

[BeforeScenario]
public void BeforeScenario()
{
    var defaultStringValueRetriever = Service.Instance.ValueRetrievers.FirstOrDefault(vr => vr is TechTalk.SpecFlow.Assist.ValueRetrievers.StringValueRetriever);
    if (defaultStringValueRetriever != null)
    {
        Service.Instance.UnregisterValueRetriever(defaultStringValueRetriever);
        Service.Instance.RegisterValueRetriever(new StringValueRetriver());
    }
}

I don't believe there is a special syntax for null and I think you'll have to just handle the conversion yourself. The value retrievers have been revised in the v2 branch and you might be able to handle this by deregistering the standard string value retriever and registering your own implementation which looks for some special syntax and returns null.

In the current 1.9.* version though I think you'll just have to check for empty string and return null yourself.

From SpecFlow 3 on-wards, in your Steps class, you can just put the following code. And in the feature file just put null value like this. Now when you use the CreateSet function then it will be deserialized correctly.

Id | Value
1  | <null> 

[Binding]
public static class YourStepClass
{
    [BeforeTestRun]
    public static void BeforeTestRun()
    {
        Service.Instance.ValueRetrievers.Register(new NullValueRetriever("<null>"));
    }
}