LAMA5002: A non-nullable contract is applied to a nullable type


Severity	Warning
Reported by	Metalama.Patterns.Contracts
Message	`The [<AspectType>] contract has been applied to '<Declaration>', but its type is nullable.`

Cause

The NotNullAttribute and RequiredAttribute contracts throw an exception when the target is assigned null. Metalama reports LAMA5002 when one of these contracts is applied to a field, property, or parameter whose type is nullable, for instance a string? or an int?. The declaration and the contract contradict each other: the type says that null is a valid value, but the contract rejects it.

The warning isn't reported for declarations whose type is a generic type parameter. The contract still applies and generates its null check.

In the following example, the Email property is declared as string?, but it has a [NotNull] contract.

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Source Code

1using Metalama.Patterns.Contracts;
2

3namespace Doc.LAMA5002.Error;
4
5public class Customer
6{
7    // Warning: the [NotNull] contract contradicts the nullable type.
8    [NotNull]


    Warning LAMA5002: The [NotNull] contract has been applied to 'Customer.Email', but its type is nullable.

9    public string? Email { get; set; }
10}
11

Transformed Code

1using System;
2using Metalama.Patterns.Contracts;
3
4namespace Doc.LAMA5002.Error;
5
6public class Customer
7{
8    private string? _email;
9
10    // Warning: the [NotNull] contract contradicts the nullable type.
11    [NotNull]
12    public string? Email
13    {
14        get
15        {
16            return _email;
17        }
18
19        set
20        {
21            if (value == null!)
22            {
23                throw new ArgumentNullException("value", "The 'Email' property must not be null.");
24            }
25
26            _email = value;
27        }
28    }
29}
30

Resolution

Do one of the following:

If null isn't a valid value, make the type non-nullable, as in the following example. The C# compiler then also warns callers that pass a possibly null value.
If null is a valid value, remove the contract.
If you need nullable types with non-nullable contracts, disable the warning by setting the WarnOnNotNullableOnNullable property of ContractOptions to false from a fabric, as described in Configuring contracts.