Code Contracts and MVVM

I have an MVVM project in C # and I want to use code contracts in it. So this is my scenario: Interface:

public interface IC042_Model
{
    void Save(C042 entity);
    void Delete(C042 entity);
}

Then I have an abstract class for contracts:

[ContractClassFor(typeof(IC042_Model))]
internal abstract class C042_Model_Contracts : IC042_Model
{
    public void Save(C042 entity)
    {
        Contract.Requires(entity != null);
    }

    public void Delete(C042_CondicaoPagamento entity)
    {
        Contract.Requires(entity != null);
    }
}

In another project, my model implements an interface, and if I call this.Save (zero), a warning is generated in any method. In my ViewModel, if I call the same method above: this.Save (null), no warning is generated, but when I run the application, the above line throws a Contract exception.

Is there something wrong with my approach?

Thanks in advance.

I made another example, which, I think, will be easier for everyone to understand:

I created the following class in a class library project:

public static class StringExtensions
{
    public static string TrimAfter(string value, string suffix)
    {
        Contract.Requires(suffix != (string)null);
        Contract.Requires(!string.IsNullOrEmpty(suffix));
        Contract.Requires(value != null);

        var index = value.IndexOf(suffix);

        if (index < 0)
            return value;

        return value.Substring(0, index);
    }
}

When I call it from a WPF project, as shown below:

CodeDigging.StringExtensions.TrimAfter(null, null);

I do not receive a warning that the contracts are not a complete field.

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