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SW001: Avoid implicit DateTime → DateTimeOffset conversion

Overview​

Rule ID: SW001
Category: Usage
Severity: Warning
Status: Enabled by default

Description​

This rule flags implicit conversions from System.DateTime to System.DateTimeOffset. These implicit conversions can hide important semantic information about the offset and Kind properties of the DateTime value, leading to subtle bugs and unexpected behavior.

When This Rule Triggers​

The analyzer reports a diagnostic when:

  • An implicit conversion from DateTime to DateTimeOffset is detected in your code
  • The conversion is user code (not compiler-generated)

Why This Matters​

The Problem​

When you implicitly convert a DateTime to DateTimeOffset:

DateTime dt = DateTime.Now;
DateTimeOffset dto = dt;  // ⚠️ Implicit conversion - SW001

The conversion doesn't preserve or communicate intent about:

  • What the original DateTime's Kind property is (Local, UTC, Unspecified)
  • What offset should be assumed for the DateTimeOffset

This can lead to:

  • Incorrect timezone handling
  • Data loss or corruption when persisting values
  • Subtle bugs that are hard to diagnose

How to Fix​

Make the conversion explicit using one of the DateTimeOffset constructors:

For Local DateTime​

DateTime dt = DateTime.Now;
// ✅ Explicit conversion showing local time zone intent
DateTimeOffset dto = new DateTimeOffset(dt);

For UTC DateTime​

DateTime dt = DateTime.UtcNow;
// ✅ Explicit conversion showing UTC intent
DateTimeOffset dto = new DateTimeOffset(dt, TimeSpan.Zero);

For DateTime with specific offset​

DateTime dt = GetDateTime();
TimeSpan offset = TimeSpan.FromHours(-5);
// ✅ Explicit conversion with explicit offset
DateTimeOffset dto = new DateTimeOffset(dt, offset);

For nullable DateTime​

DateTime? dt = DateTime.Now;
// ✅ Handle nullable explicitly
DateTimeOffset? dto = dt.HasValue 
    ? new DateTimeOffset(dt.Value) 
    : null;

Configuration​

This rule is enabled by default. To disable it, add to your .editorconfig:

[*.cs]
dotnet_diagnostic.SW001.severity = none

To change severity to error:

[*.cs]
dotnet_diagnostic.SW001.severity = error
  • None

See Also​