net 2 上的 C# 多线程，firebird 数据库取消并重新启动，如何？

Question

In winform app, I have DataGridView and thread which is loading data from database when user press button.

This loading is done on another thread, I don't know how to make the following scenario:

when user press another button, the same procedure starts, with different query, cancelling previous thread and loading data only from second time.

Currently it works in such way, that DataGridView is populated with second query, but after few seconds its repopulated with first query. And it shouldn't, first query should be in some way cancelled.

How to do that?

Answer 1

Below is a basic example how you can achieve the desired behavior. There is no cancelling database query, but there is ignoring results that is not relevant anymore.

public static IEnumerable<Row> GetRowsForTheFirstButton()
{
    var token = GridResultsSync.GetCurrentToken();

    // retrieving data
    ...

    if (GridResultsSync.IsTokenRelevant(token))
        return result;
    return null;
}
// exactly the same code for the second method
public static IEnumerable<Row> GetRowsForTheSecondButton()
{
    var token = GridResultsSync.GetCurrentToken();

    // retrieving data
    ...

    if (GridResultsSync.IsTokenRelevant(token))
        return result;

    return null;
}

...
class GridResultsSync
{
    static int _count = 0;
    public static int GetCurrentToken()
    {
        return Interlocked.Increment(ref _count);
    }
    public static bool IsTokenRelevant(int token)
    {
        var currentCount = Interlocked.Increment(ref _count);
        return currentCount - token  <=  2;
    }
}

When you call GetRowsForTheFirstButton() and GetRowsForTheSecondButton() then you check if the result is not null , then you bind the result to grid, if the result is null then do nothing.

If you really want to cancel a query to DB then you may consider a little more complex approach. It can be done using SqlCommand.Cancel Method or if you are using Entity Framework you can use CancellationToken .