Fixed: "error CS8629: Nullable value type may be null" in C#

error CS8629: Nullable value type may be null. I'd used request.UnitPrice.Value on a decimal?, and the project treats warnings as errors, so the build stopped. Validation guarantees the value is there before my code runs, but the compiler can't see validation. You tell it what you know, in one of three ways.

What I Was Doing

In a .NET 10 Web API, a request record has a nullable price, so that [Required] can catch a missing one:

src/Dewiride.Inventory.Api/Features/Products/ProductContracts.cs (excerpt)

[Required, Money] decimal? UnitPrice,

The entity's UnitPrice is a plain decimal, so the endpoint copies the value across:

src/Dewiride.Inventory.Api/Features/Products/ProductEndpoints.cs (excerpt)

var product = new Product
{
    // …
    UnitPrice = request.UnitPrice.Value,
    // …
};

The solution has <Nullable>enable</Nullable> and <TreatWarningsAsErrors>true</TreatWarningsAsErrors>.

What I Expected

Build succeeded. The value can't be null there: validation runs first.

What I Got

Output (the path is shortened)

ProductEndpoints.cs(41,25): error CS8629: Nullable value type may be null.

What I Checked

What the compiler knows. C#'s nullable warnings page lists CS8629 among the warnings about a possible null. The compiler follows checks in the same method, such as if (x is null), and an attribute on a record in another file isn't one of those.

Why it's an error. It's a warning, and TreatWarningsAsErrors makes it an error. Turning that setting off isn't the fix: How to Treat Warnings as Errors explains why it's there.

Whether the value can really be null there. With [Required] and AddValidation, a request without a price gets a 400 before the endpoint runs:

Response (POST with no unitPrice)

HTTP/1.1 400 Bad Request
Content-Type: application/problem+json

{
  "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "errors": {
    "UnitPrice": [
      "The UnitPrice field is required."
    ]
  },
  "traceId": "00-3ef294d4f4809b6060ef90706edb6747-0c75ca43a6d85633-00"
}

So in this case the compiler is being careful about something that can't happen.

Why It Happens

.Value throws InvalidOperationException when a nullable value type has no value, so with nullable analysis on, the compiler warns unless it can prove there's a value. It can only see code in the method, not what ASP.NET Core does before calling it.

The Fix

Pick the one that says what you mean:

src/Dewiride.Inventory.Api/Features/Products/ProductEndpoints.cs (excerpt)

UnitPrice = request.UnitPrice.GetValueOrDefault(),

• GetValueOrDefault() returns the value, or 0 if there's none, and never throws. It's the one I used: validation has already ruled out null.
• request.UnitPrice!.Value tells the compiler "trust me". It still throws if you're wrong, which can be what you want when null would be a bug.
• request.UnitPrice ?? 0m says what happens if it's missing. Use it when there's a sensible default.

After the Fix

Output

Build succeeded.
    0 Warning(s)
    0 Error(s)

A real price still gets through correctly, and a missing one still answers 400.

Common Mistakes

Mistake	What happens	Do this instead
Turning off TreatWarningsAsErrors	Every other warning slips through too	Fix this line
Adding #pragma warning disable	It hides the warning, and the risk	Say what you mean in the code
Making the request property a plain decimal	A missing price silently becomes 0	Keep decimal? with [Required]
Using GetValueOrDefault() without validation	A missing value quietly becomes 0	Use it only when null is ruled out

Troubleshooting

Symptom	Likely cause	Fix
CS8629 on .Value	The compiler can't prove there's a value	Use one of the three above, or check first
CS8602: Dereference of a possibly null reference	The same idea, for a reference type	Check for null, or use !
CS0266: Cannot implicitly convert type 'decimal?' to 'decimal'	You left .Value out	See that post
InvalidOperationException: Nullable object must have a value at runtime	.Value with ! on a real null	Validate first, or use ??

FAQ

Does a check in the same method remove the warning?

Yes. After if (request.UnitPrice is null) return …;, the compiler knows there's a value, so .Value compiles without a warning.

Which option is safest?

They're all safe here, because validation runs first. Without validation, ?? or a check is the honest choice.

Conclusion

error CS8629: Nullable value type may be null means the compiler can't prove a decimal? has a value where you read .Value. It can't see validation attributes. Use GetValueOrDefault(), !.Value or ??, whichever says what you mean, and keep TreatWarningsAsErrors on.

Additional Resources

• Resolve nullable warnings — every nullable warning, and what to do.
• Nullable value types — .Value, GetValueOrDefault and ??.

Related

• Fixed: "error CS0266: Cannot implicitly convert type 'decimal?' to 'decimal'" in C# (Nullable Decimal)
• Fixed: A Missing Number in a JSON Request Becomes 0 in an ASP.NET Core Minimal API
• How to Treat Warnings as Errors in .NET C# Projects (TreatWarningsAsErrors)