Unable to cast object of type 'System.DBNull' to type 'System.String`

Question

I got the above error in my app. Here is the original code

public string GetCustomerNumber(Guid id)
{
     string accountNumber = 
          (string)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidmyApp, 
                          CommandType.StoredProcedure, 
                          "GetCustomerNumber", 
                          new SqlParameter("@id", id));
     return accountNumber.ToString();
 }

I replaced with

public string GetCustomerNumber(Guid id)
{
   object accountNumber =  
          (object)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidCRM, 
                                CommandType.StoredProcedure, 
                                "spx_GetCustomerNumber", 
                                new SqlParameter("@id", id));
    if (accountNumber is System.DBNull)
    {
       return string.Empty;
    }
    else
    {
       return accountNumber.ToString();
    }
}

Is there a better way around this?

Answer 1

With a simple generic function you can make this very easy. Just do this:

return ConvertFromDBVal<string>(accountNumber);

using the function:

public static T ConvertFromDBVal<T>(object obj)
{
    if (obj == null || obj == DBNull.Value)
    {
        return default(T); // returns the default value for the type
    }
    else
    {
        return (T)obj;
    }
}

Answer 2

A shorter form can be used:

return (accountNumber == DBNull.Value) ? string.Empty : accountNumber.ToString()

EDIT: Haven't paid attention to ExecuteScalar. It does really return null if the field is absent in the return result. So use instead:

return (accountNumber == null) ? string.Empty : accountNumber.ToString() 

Answer 3

ExecuteScalar will return

• null if there is no result set
• otherwise the first column of the first row of the resultset, which may be DBNull.

If you know that the first column of the resultset is a string, then to cover all bases you need to check for both null and DBNull. Something like:

object accountNumber = ...ExecuteScalar(...);
return (accountNumber == null) ? String.Empty : accountNumber.ToString();

The above code relies on the fact that DBNull.ToString returns an empty string.

If accountNumber was another type (say integer), then you'd need to be more explicit:

object accountNumber = ...ExecuteScalar(...);
return (accountNumber == null || Convert.IsDBNull(accountNumber) ?     
         (int) accountNumber : 0;

If you know for sure that your resultset will always have at least one row (e.g. SELECT COUNT(*)...), then you can skip the check for null.

In your case the error message "Unable to cast object of type ‘System.DBNull’ to type ‘System.String`" indicates that the first column of your result set is a DBNUll value. This is from the cast to string on the first line:

string accountNumber = (string) ... ExecuteScalar(...);

Marc_s's comment that you don't need to check for DBNull.Value is wrong.

Answer 4

You can use C#'s null coalescing operator

return accountNumber ?? string.Empty;

Answer 5

This is the generic method that I use to convert any object that might be a DBNull.Value:

public static T ConvertDBNull<T>(object value, Func<object, T> conversionFunction)
{
    return conversionFunction(value == DBNull.Value ? null : value);
}

usage:

var result = command.ExecuteScalar();

return result.ConvertDBNull(Convert.ToInt32);

shorter:

return command
    .ExecuteScalar()
    .ConvertDBNull(Convert.ToInt32);

Answer 6

There is another way to workaround this issue. How about modify your store procedure? by using ISNULL(your field, "") sql function , you can return empty string if the return value is null.

Then you have your clean code as original version.

Answer 7

I suppose you can do it like this:

string accountNumber = DBSqlHelperFactory.ExecuteScalar(...) as string;

If accountNumber is null it means it was DBNull not string :)

Answer 8

String.Concat transforms DBNull and null values to an empty string.

public string GetCustomerNumber(Guid id)
{
   object accountNumber =  
          (object)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidCRM, 
                                CommandType.StoredProcedure, 
                                "spx_GetCustomerNumber", 
                                new SqlParameter("@id", id));

    return String.Concat(accountNumber);

 }

However, I think you lose something on code understandability

Answer 9

Since I got an instance which isn't null and if I compared to DBNULL I got Operator '==' cannot be applied to operands of type 'string' and 'system.dbnull' exeption, and if I tried to change to compare to NULL, it simply didn't work ( since DBNull is an object) even that's the accepted answer.

I decided to simply use the 'is' keyword. So the result is very readable:

data = (item is DBNull) ? String.Empty : item

Answer 10

Since I just had a similar error myself and this is the first hit on google. If you do not have your properties nullable configured to match your database, the same error can occur.

If you just need a quick & ugly fix, you can add:

#nullable disable

at the start of your model class.

Answer 11

based on answer from @rein

public static class DbDataReaderExtensions
{
    public static TObjProp Get<TObj, TObjProp>(
        this DbDataReader reader,
        Expression<Func<TObj, TObjProp>> expression)
    {
        MemberExpression member = expression.Body as MemberExpression;
        string propertyName = member.Member.Name;

        //PropertyInfo propInfo = member.Member as PropertyInfo;

        var recordOrdinal = reader.GetOrdinal(propertyName);
        var obj = reader.GetValue(recordOrdinal);

        if (obj == null || obj == DBNull.Value)
        {
            return default(TObjProp);
        }
        else
        {
            return (TObjProp)obj;
        }
    }
}

Given:

public class MyClass
{
    public bool? IsCheckPassed { get; set; }
}

Use as:

var test = reader.Get<MyClass, bool?>(o => o.IsCheckPassed);

or, if you hardcode class type in exception method:

var test = reader.Get(o => o.IsCheckPassed);

p.s. I haven't figured yet how to make generics implicit without sacrificing code length.. fee free to comment and suggest improvements

Full example:

public async Task<MyClass> Test(string connectionString) {
    var result = new MyClass();
    
    await using var con = new SQLiteConnection(connectionString);
    con.Open();

    await using var cmd = con.CreateCommand();
    cmd.CommandText = @$"SELECT Id, IsCheckPassed FROM mytable";
    
    var reader = await cmd.ExecuteReaderAsync();
    while (reader.Read()) {
        // old, not working! Throws exception!
        //bool? isCheckPassed1 = reader.GetBoolean(reader.GetOrdinal("IsCheckPassed"));
        
        // old, working, but too long (also if you have like 20 properties then all the more reasons to refactor..)
        bool? isCheckPassed2 = null;
        bool? isCheckPassed2Temp = reader.GetValue(reader.GetOrdinal("IsCheckPassed"));
        if (isCheckPassed2Temp != null && isCheckPassed2Temp != DBNull.Value)
            isCheckPassed2 = (bool?)isCheckPassed2Temp;
        
        // new
        var isCheckPassed3 = reader.Get<MyClass, bool?>(o => o.IsCheckPassed);
        // repeat for 20 more properties :)
        
        result.IsCheckPassed = isCheckPassed3;
    }
    
    return result;
}

Solution will work for as long as table column names match property names of the class. And might not be production-grade performance wise, so use or modify at your own risk :)

Answer 12

A more concise approach using more recent C# syntax and also accounting for nullable types:

private static T? FromDbNull<T>(object? obj) => 
    obj == null || obj == DBNull.Value ? default : (T)obj;

Can be used with a data reader as follows:

        while (reader.Read())
        {
            var newObject = new SomeObject(
                FromDbNull<string?>(reader["nullable_field_1"]),
                FromDbNull<string?>(reader["nullable_field_2"]),
                FromDbNull<string?>(reader["nullable_field_3"]), 
                FromDbNull<double?>(reader["nullable_field_4"])
            );
            
            response.Add(newObject);
        }

Answer 13

Convert it Like

string s = System.DBNull.value.ToString();