ReSharper - Possible Null Assignment when using Microsoft.Contracts

Question

Is there any way to indicate to ReSharper that a null reference won't occur because of Design-by-Contract Requires checking? For example, the following code will raise the warning (Possible 'null' assignment to entity marked with 'NotNull' attribute) in ReSharper on lines 7 and 8:

private Dictionary<string, string> _Lookup = new Dictionary<string, string>();

public void Foo(string s)
{
    Contract.Requires(!String.IsNullOrEmpty(s));

    if (_Lookup.ContainsKey(s))
        _Lookup.Remove(s);
}

What is really odd is that if you remove the Contract.Requires(...) line, the ReSharper message goes away.

Update

I found the solution through ExternalAnnotations which was also mentioned by Mike below. Here's an example of how to do it for a function in Microsoft.Contracts:

• Create a directory called Microsoft.Contracts under the ExternalAnnotations ReSharper directory.
• Next, Create a file called Microsoft.Contracts.xml and populate like so:

<assembly name="Microsoft.Contracts">
    <member name="M:System.Diagnostics.Contracts.Contract.Requires(System.Boolean)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
</assembly>

• Restart Visual Studio, and the message goes away!

Answer 1

Note: as of the current R# 8.0 EAP, this functionality is included.

---

Here's the solution for the current (i.e. .NET 4.0) version of Code Contracts:

Inside ...\ExternalAnnotations\mscorlib\Contracts.xml, add the following:

<assembly name="mscorlib">
    <member name="M:System.Diagnostics.Contracts.Contract.Assert(System.Boolean)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Assert(System.Boolean, System.String)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Assume(System.Boolean)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Assume(System.Boolean, System.String)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Requires(System.Boolean)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Requires``1(System.Boolean)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Requires(System.Boolean,System.String)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Requires``1(System.Boolean,System.String)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Invariant(System.Boolean)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
    <member name="M:System.Diagnostics.Contracts.Contract.Invariant(System.Boolean,System.String)">
        <attribute ctor="M:JetBrains.Annotations.AssertionMethodAttribute.#ctor"/>
        <parameter name="condition">
            <attribute ctor="M:JetBrains.Annotations.AssertionConditionAttribute.#ctor(JetBrains.Annotations.AssertionConditionType)">
                <argument>0</argument>
            </attribute>
        </parameter>
    </member>
</assembly>