How to return null from a Dapper query rather than

问题:

I'm using Dapper for some read-only database calls via a stored procedure. I've got a query that will either return 1 row or nothing.

I'm using Dapper like this:

using (var conn = new SqlConnection(ConnectionString))
{
    conn.Open();

    return conn.Query<CaseOfficer>("API.GetCaseOfficer", 
        new { Reference = reference }, 
        commandType: CommandType.StoredProcedure).FirstOrDefault();
}

The returned CaseOfficer object looks like this:

public class CaseOfficer
{
    public string Title { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string Telephone { get; set; }
}

This is then returned through a ASP.NET Web API application as JSON.

When the stored procedure returns a result I get the following:

{
    title: "Mr",
    firstName: "Case",
    lastName: "Officer",
    email: "[e-mail]",
    telephone: "01234 567890"
}

But when it returns nothing I get:

{
    title: null,
    firstName: null,
    lastName: null,
    email: null,
    telephone: null
}

How can I get Dapper to return null (so I can check and respond with 404), rather than the default(CaseOfficer)?

回答1:

If your SP doesn't return a row, then dapper won't return a row; so first thing to check: did your SP perhaps return an empty row? A row of all nulls ? Or did it return 0 rows?

Now, assuming no row was returned, FirstOrDefault (the standard LINQ-to-Objects thing) will return null if CaseOfficer is a class, or a default instance if CaseOfficer is a struct. So next: check CaseOfficer is a class (I can't think of any sane reason that would be a struct).

But: dapper with FirstOrDefault will generally already do what you want.

回答2:

You can set default properties.

For example:

public class BaseEntity
{
    public BaseEntity()
    {
        if (GetType().IsSubclassOf(typeof(BaseEntity)))
        {
            var properties = GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance);

            foreach (var property in properties)
            {
                // Get only string properties
                if (property.PropertyType != typeof (string))
                {
                    continue;
                }

                if (!property.CanWrite || !property.CanRead)
                {
                    continue;
                }

                if (property.GetGetMethod(false) == null)
                {
                    continue;
                }
                if (property.GetSetMethod(false) == null)
                {
                    continue;
                }

                if (string.IsNullOrEmpty((string) property.GetValue(this, null)))
                {
                    property.SetValue(this, string.Empty, null);
                }
            }
        }
    }
}

public class CaseOfficer : BaseEntity
{
    public string Title { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string Telephone { get; set; }
}

Now, CaseOfficer got auto-properties