Make the Nullable<T>.Value exception message actionable

[jeffhandley]

Fixes #126560

Problem

InvalidOperation_NoValue — the message thrown by Nullable<T>.Value when there's no value — reads:

Nullable object must have a value.

As #126560 points out, this states an obligation on the object's state rather than naming the invalid action. It reads as "you must assign a value", when the actual fix is almost always the opposite: don't read .Value without checking .HasValue first. It also never mentions which member was accessed.

Change

-Nullable object must have a value.
+Cannot read the Value property of a Nullable object that has no value. Check HasValue before reading Value.

This follows the wording style already used in the repo for comparable "you called this on something that has no value" failures:

• SqlMisc_NullValueMessage (System.Data.Common) — "Data is Null. This method or property cannot be called on Null values."
• ReflectionModel_ExportNotReadable (System.ComponentModel.Composition) — "Cannot get the value of property '{0}', because the member is not readable. The property must have an accessible getter."
• NoResultOnFailed (System.Text.RegularExpressions) — "Result cannot be called on a failed Match."

The issue also floated "Cannot read the Value property of a null Nullable object." I avoided "null Nullable object" because Nullable<T> is a struct and is never null — "that has no value" keeps the same cadence without the imprecision. The second sentence names the check to perform, per "DO ensure that exception messages are clear and actionable" (docs/coding-guidelines/framework-design-guidelines-digest.md).

The resource name is unchanged.

Scope

InvalidOperation_NoValue is not shared with any unrelated call site — all definitions and references are Nullable<T>.Value semantics:

	Path
def	src/libraries/System.Private.CoreLib/src/Resources/Strings.resx
def	src/coreclr/nativeaot/System.Private.Reflection.Execution/src/Resources/Strings.resx
use	ThrowHelper.ThrowInvalidOperationException_InvalidOperation_NoValue — sole caller is the Nullable<T>.Value getter
use	CustomMethodMapper.Nullable.cs — NativeAOT reflection-invoke shim for Nullable<T>.Value

Both .resx definitions are updated. Mono shares the libraries CoreLib .resx; there is no third copy. No test asserts the text, and the NativeAOT FrameworkStrings string-pinning smoke test pins other resources.

Per docs/coding-guidelines/breaking-change-rules.md, changing the text of an error message is not a breaking change ("users should not rely on these text messages, and they change anyways based on culture").

Validation

• Built System.Private.CoreLib (build.cmd clr.corelib -c Release) — clean.
• Confirmed against the produced System.Private.CoreLib.dll that the new string is embedded and the old one is gone, so SR.InvalidOperation_NoValue still resolves.
• The NativeAOT System.Private.Reflection.Execution .resx was not compiled locally — that project currently fails on a pre-existing missing generated AsmOffsets.cs requiring a full NativeAOT native build. The edit there is identical in shape and doesn't change the resource name. CI covers it.

Note

This pull request description was generated by GitHub Copilot.

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[Copilot]

Pull request overview

Updates the InvalidOperation_NoValue resource string to make the Nullable<T>.Value failure message describe the invalid action (reading Value when there is no value) and provide a concrete next step (check HasValue) while keeping the resource key unchanged across CoreLib and NativeAOT.

Changes:

• Replaces the InvalidOperation_NoValue text in System.Private.CoreLib resources with an actionable message referencing Value/HasValue.
• Mirrors the same updated text in the NativeAOT System.Private.Reflection.Execution resources to keep behavior consistent across runtimes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.CoreLib/src/Resources/Strings.resx	Updates InvalidOperation_NoValue message used by Nullable<T>.Value via ThrowHelper.
src/coreclr/nativeaot/System.Private.Reflection.Execution/src/Resources/Strings.resx	Updates the NativeAOT InvalidOperation_NoValue message used by the reflection invoke shim for Nullable<T>.Value.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/coreclr/nativeaot/System.Private.Reflection.Execution/src/Resources/Strings.resx:125

• This change removes multiple unrelated resource entries (e.g., Arg_EmptyArray, MissingMember, InvalidOperation_NoValue) from this .resx. Even if they were unused, this is a large scope expansion for a PR that appears to be about improving Nullable.Value’s exception message, and it makes it harder to review/attribute behavioral changes. Please revert these unrelated deletions (or split them into a separate cleanup PR with rationale).

  <data name="Reflection_InsufficientMetadata_EdbNeeded" xml:space="preserve">
    <value>'{0}' is missing native code or metadata. This can happen for code that is not compatible with trimming or AOT. Inspect and fix trimming and AOT related warnings that were generated when the app was published. For more information see https://aka.ms/nativeaot-compatibility</value>
  </data>
  <data name="Arg_HTCapacityOverflow" xml:space="preserve">
    <value>Hashtable's capacity overflowed and went negative. Check load factor, capacity and the current size of the table.</value>
  </data>

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (2)

src/coreclr/nativeaot/System.Private.Reflection.Execution/src/Resources/Strings.resx:124

• The PR description says the NativeAOT System.Private.Reflection.Execution Strings.resx definition for InvalidOperation_NoValue is updated, but this diff instead deletes that entry (and several other resource keys). If the extra deletions are intentional cleanup, please split them into a separate PR and/or update the PR description; otherwise, consider restoring the removed resource entries.

  <data name="Reflection_InsufficientMetadata_EdbNeeded" xml:space="preserve">
    <value>'{0}' is missing native code or metadata. This can happen for code that is not compatible with trimming or AOT. Inspect and fix trimming and AOT related warnings that were generated when the app was published. For more information see https://aka.ms/nativeaot-compatibility</value>
  </data>
  <data name="RFLCT_Targ_StatFldReqTarg" xml:space="preserve">
    <value>Non-static field requires a target.</value>

src/coreclr/nativeaot/System.Private.Reflection.Execution/src/System.Private.Reflection.Execution.csproj:70

• This PR is described as a targeted update to the Nullable.Value exception message, but it also removes the linked HashHelpers.cs compile item from System.Private.Reflection.Execution. Even if unused, this is an unrelated change that makes the PR harder to review/safely backport; consider reverting this in this PR (or documenting why it’s needed).

    <Compile Include="$(CompilerCommonPath)\Internal\LowLevelLinq\LowLevelEnumerable.ToArray.cs">
      <Link>Internal\LowLevelLinq\LowLevelEnumerable.ToArray.cs</Link>
    </Compile>
  </ItemGroup>