250117 ‐ debugging field change - cywongg/2025 GitHub Wiki

Yes, in JetBrains Rider (or any modern debugger), you can use breakpoints and data breakpoints (also sometimes known as "watchpoints") to debug and monitor when a field or collection is being changed. Here's a detailed guide to help you debug the issue:

1. Understand the Problem

If a field in a collection is unexpectedly 0 when it shouldn't be, you might suspect:

A threading issue (e.g., race condition or improper synchronization).
The value is being overwritten by another part of the code.
The field is being improperly initialized or reset.

Debugging this requires monitoring when the field is accessed or modified.

2. Use Breakpoints in Rider

a. Set a Breakpoint on Code Setting the Field

If you know where the field is being set (e.g., via a property setter or directly in code), place a breakpoint on that line.
Run the debugger in Rider.
When the breakpoint is hit, inspect the call stack and threads to see how and where the field is being updated.

b. Break on All Setters or Modifications

If the field is being modified in multiple places and you don't know exactly where, you can:

Search for all instances of the field being modified in your code using Ctrl + Shift + F (Find in Files).
Place breakpoints on all the relevant lines.
Run the debugger and observe when the breakpoints are hit.

3. Use Conditional Breakpoints

If the field is modified frequently but you only care about specific situations (e.g., when the value becomes 0):

Right-click the breakpoint in Rider.
Select Edit Breakpoint.
Add a condition, such as:
```
myField == 0
```
The debugger will only stop when the condition is true.

4. Monitor Field Changes with a Data Breakpoint

Rider itself doesn't yet fully support traditional data breakpoints (as of 2023), but there are workarounds for this. If you're debugging in .NET, you can use the following approaches:

a. Use a Property Instead of a Field

Convert the field to a property with a custom setter to monitor changes:

private int _myField;
public int MyField
{
    get => _myField;
    set
    {
        _myField = value;
        Console.WriteLine($"MyField was set to: {_myField}");
    }
}

This logs whenever the property is set. You can also add a breakpoint inside the setter.

b. Use `Debugger.Break` in the Setter

If you'd like the debugger to halt execution immediately when a field changes:

private int _myField;
public int MyField
{
    get => _myField;
    set
    {
        System.Diagnostics.Debugger.Break(); // Triggers the debugger
        _myField = value;
    }
}

c. Manually Observe Collection Changes

If you're debugging a collection, you can override the collection type (e.g., by using ObservableCollection or a custom wrapper):

public class DebugCollection<T> : List<T>
{
    public new T this[int index]
    {
        get => base[index];
        set
        {
            Console.WriteLine($"Index {index} was set to {value}");
            base[index] = value;
        }
    }
}

Replace your collection with DebugCollection<T> to monitor changes.

5. Check for Threading Issues

Threading issues often cause unexpected values. To debug this:

a. Enable Concurrent Execution Debugging

Run the program in Debug mode.
In Rider, open the Threads view during debugging to see all active threads.
Look for unexpected threads accessing the field or collection.

b. Add Locks or Synchronization

To test if threading is the issue, temporarily wrap the field access in a lock:

private readonly object _lock = new object();
private int _myField;

public int MyField
{
    get
    {
        lock (_lock)
        {
            return _myField;
        }
    }
    set
    {
        lock (_lock)
        {
            _myField = value;
        }
    }
}

If this resolves the issue, you likely have a race condition.

c. Use `Interlocked` for Atomic Operations

For simple numeric fields, consider using Interlocked methods to ensure atomic updates:

System.Threading.Interlocked.Exchange(ref _myField, newValue);

6. Debug Collection Modifications

If the field is part of a collection:

Check for Iteration Issues: Ensure you're not modifying the collection while iterating through it.
Use Wrappers: Wrap the collection with a debugging implementation (e.g., DebugCollection as shown earlier).
Monitor Events: If using ObservableCollection, listen to the CollectionChanged event to see when changes occur.

7. Analyze the Call Stack and Threads

When the breakpoint is hit:

Inspect the Call Stack (in the Debug view) to see what code is executing.
Check the Threads view to ensure the modification is happening on the correct thread.

By using these techniques, you can determine exactly why and when the field's value is changing, and whether threading is causing the issue. Let me know if you need more clarification!