_callReportCode = reader["Call Report Code"].ToString();
I am attempting to handle the possibility for the object I am calling ToString on to be NULL.
I am going to be using the above statement with several variables and I dont want to make an individual try/catch for each one... what is the best way to do null checking for strings.
Other datatypes ive been doing this:
int.TryParse(reader["Account Number"].ToString(), out _accountNumber);
In this code "reader" refers to a SqlDataReader but thats not really important for this question.
Use the null-coalescing operator: ??
callReportCode = (reader["Call Report Code"] ?? "").ToString();
If the data in your field is DBNull.Value (rather than null), this will still work, because DBNull.Value is not null, so the ?? won't be used, and DBNull.Value.ToString() is "", which is what you'd want.
Convert.ToString(reader["Call Report Code"]);
It will return string.Empty if the value is null.
Source: http://msdn.microsoft.com/en-us/library/astxcyeh.aspx
Update: it also works with DBNull, I've just verified.
Update 2: I decided to bring a more complete test here, just to be sure:
DBNull dbNull = null;
DBNull dbNullEmpty = DBNull.Value;
string stringNull = null;
string stringEmpty = string.Empty;
var outcome1 = Convert.ToString(dbNull);//Empty string
var outcome2 = Convert.ToString(dbNullEmpty);//Empty string
var outcome3 = Convert.ToString(stringNull);//NULL
var outcome4 = Convert.ToString(stringEmpty);//Empty string
If your string is nullable, you need to check the value returned from the SqlDataReader against DBNull.Value:
_callReportCode = reader["Call Report Code"] as string;
If the object returned by reader["Call Report Code"] is not a string, it's DBNull.Value, so the as cast is going to set the value of _callReportCode to null as well.
If you must set the string to a non-null in case the database value is missing, add ??, like this:
_callReportCode = (reader["Call Report Code"] as string) ?? string.Empty;
My suggestion is to never convert ToString when the data isn't a string, and if the data is already a string, then calling ToString is redundant, and a cast is all that's required.
I am making an assumption that the datatype in the database is integer, in which case, you can use a nullable int.
int? accountNumber = reader["Account Number"] == DBNull.Value ? null : (int?)reader["Account Number"];
I have made an extension method to do just this thing.
public static class SqlDataReaderExtensions
{
public static T Field<T>(this SqlDataReader reader, string columnName)
{
object obj = reader[columnName];
if (obj == null)
{
throw new IndexOutOfRangeException(
string.Format(
"reader does not contain column: {0}",
columnName
)
);
}
if (obj is DBNull)
{
obj = null;
}
return (T)obj;
}
}
Usage
int? accountType = reader.Field<int?>("Account Number"); // will return NULL or the account number.
The easiest way I have found is
_callReportCode = reader["Call Report Code"] + "";
i have some easiest and common Method.
public static string ToNULLString(this string Values)
{
if (string.IsNullOrEmpty(Values))
{
return "";
}
else
{
return Values.ToString();
}
}
use in C#
string item = null;
string value = item.ToNULLString();
you could create a method that you call when you want to make the check.
This way you have to type the try catch only once...
or you can create an extension method for string class to do this
_callReportCode = Convert.ToString(reader["Call Report Code"]) should ensure there are no null there.
Use following line of code:
_callReportCode = String.IsNullorEmpty(reader["Call Report Code"]) ?
String.Empty :
reader["Call Report Code"].ToString();
instead of the following line:
_callReportCode = reader["Call Report Code"].ToString();
I like using a combination of the null-coalescing operator and the null conditional operator:
string nn = MyObject.myNullableVar?.ToString() ?? "";
it's basically the same as this
string ss = (MyObject.MyNullableVar == null) ? "" : MyObject.MyNullableVar.ToString();
but shorter.
You can perform a check using String.IsNullOrEmpty() to ensure that it isn't going to be null, or you could write an extension method to perform some action if it's not null and another / nothing if it is.
Related
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?
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;
}
}
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()
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.
You can use C#'s null coalescing operator
return accountNumber ?? string.Empty;
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);
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.
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 :)
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
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
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 :)
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);
}
I use an extension to eliminate this problem for me, which may or may not be what you are after.
It goes like this:
public static class Extensions
{
public String TrimString(this object item)
{
return String.Format("{0}", item).Trim();
}
}
Note:
This extension does not return null values! If the item is null or DBNull.Value, it will return an empty String.
Usage:
public string GetCustomerNumber(Guid id)
{
var obj =
DBSqlHelperFactory.ExecuteScalar(
connectionStringSplendidmyApp,
CommandType.StoredProcedure,
"GetCustomerNumber",
new SqlParameter("#id", id)
);
return obj.TrimString();
}
Convert it Like
string s = System.DBNull.value.ToString();
I have the following property:
IDictionary<string, object> Values { get; }
The property has a few values having one of them the key "culture".
I tried to use that value and casting to a string:
String value = (String)data.Values["culture"] ?? defaultCulture;
This works when the item has some value but when it doesn't I get the error:
Unable to cast object of type 'System.Web.Mvc.UrlParameter' to type 'System.String'.
BTW, System.Web.Mvc.UrlParameter:
http://msdn.microsoft.com/en-us/library/system.web.mvc.urlparameter%28v=vs.108%29.aspx
In the debugger data.Values["culture"] has the value {}. I tested and:
var test_1 = data.Values["culture"] == null; // returns false
var test_2 = data.Values["culture"].ToString() == null; // returns false
How do I check if data.Values["culture"] has something in it or not?
Apparently, you are storing a UrlParameter in that dictionary entry. If you want to get a string out, you will need to use ToString().
The exact expression to use will depend on the type of defaultCulture. If it is a string, then
String value = data.Values["culture"] == null ? defaultCulture : data.Values["culture"].ToString();
if it is a UrlParameter, then
String value = (data.Values["culture"] ?? defaultCulture).ToString();
If you want to get the string if it indeed is a string and a default value otherwise, you can test the type of the valuer using as. Something like:
(data.Values["culture"] as string) ?? defaultCulture
Try this:
String value = (String) (data.Values["culture"] ?? defaultCulture);
The idea is to check for null before trying to cast.
did you use this?
string.IsNullOrWhiteSpace(somestring)
I am returning a scalar value from a SQL Server 2008 database:
string reason = cmd.ExecuteScalar().ToString() ?? : "";
I want to make sure that if null is returned, that reason = "" and not null.
i am getting an error on this line:
Error 3 Invalid expression term ':'
How can this be fixed?
EDIT:
thank you for the changes on the colon, now i am getting this exception on the same line:
string reason = cmd.ExecuteScalar().ToString() ?? "";
System.NullReferenceException occurred
Message="Object reference not set to an instance of an object."
Try this:
string reason = cmd.ExecuteScalar().ToString() ?? "";
BUT: this will still fail, since if .ExecuteScalar() returns a NULL, you're already causing a Null Reference Exception by calling .ToString() on that NULL value......
So I guess the ?? operator really doesn't help you here... do the "usual" dance:
object result = cmd.ExecuteScalar();
if(result != null)
{
reason = result.ToString();
}
else
{
reason = "(NULL value returned)";
}
First, you shouldn't have the : when using the ?? operator.
Second, to do what you are trying to do here without getting an error, you need to do it differently:
object objReason = cmd.ExecuteScalar();
string reason = objReason == null ? "" : objReason.ToString();
This will check whether or not your returned value is null and if it is, the second line will set reason to a blank string, otherwise it will use your returned value.
Since ExecuteScalar() returns object that might be null you should not call .ToString() since that may throw and exception.
string reason = Convert.ToString(cmd.ExecuteScalar());
This works because Convert.ToString() will convert null to string.Empty
or if you must use ?? because you really like it:
(cmd.ExecuteScalar() ?? (object)"").ToString();
Just get rid of the colon.
string reason = cmd.ExecuteScalar().ToString() ?? "";
For reference, check the MSDN page.
When using the null-coalescing operator, you don't need the colon:
string reason = cmd.ExecuteScalar().ToString() ?? "";
As others have pointed out though, ToString() would cause a NullReferenceExcpetion to be thrown anyway...so you don't gain anything here. You'd be much better off splitting this into multiple lines:
var result = cmd.ExecuteScalar();
string reason = result == null ? "" : result.ToString();
You're confusing the ? conditional operator, the syntax for which looks like this:
String x = condition ? valueIfConditionIsTrue : valueIfConditionIsFalse;
with the ?? null-coalesce operator whose syntax is as follows:
String x = possiblyNull ?? valueIfPossiblyNullIsNull;
So, apart from all that... this is the part you really want:
String reason = (cmd.ExecuteScalar() ?? "").ToString();
This takes care of your exception where ToString() was causing a null-reference exception.
Just use
string reason = cmd.ExecuteScalar() ?? "";
When looping through a DataRow and encountering types such as
DataRow dr;
dr["someString"]
dr["someInteger"]
dr["somedata"]
What's the best way to get them into their corresponding data types? dr["foo"] is just a generic object.
Also, are these able to be easily converted to nullable types? dr["someInteger"] could be null.
When reading from a DataRow, your biggest enemy is a null value. In a DataRow, when a value is null, it is not equals to null: It is equals to DBNull.Value.
if(DBNull.Value == null)
{
// Will never happen
}
Unless you know that your field cannot be null, it is not safe to cast. For example, the following example will fail if the data is DBNull:
string name = (string)dr["Name"];
If you can use the LINQ extensions, you can include the reference System.Data.DataSetExtensions and the namespace System.Data and call
string name = dr.Field<string>("Name");
If you cannot use LINQ, then you have to fall back to checking for null value with
string name = null;
if(!dr.IsNull("Name"))
name = (string)dr["Name"];
Or you could code your own Field function like this:
public static T GetValue<T>(object value)
{
if (value == null || value == DBNull.Value)
return default(T);
else
return (T)value;
}
and get your value this way:
string name = GetValue<string>(dr["Name"]);
If you can use .net 3.5, then you can use the Field extension method to more easily access the data if you know the type. An example would be:
string somestring= row.Field<string>("SomeString");
Otherwise you're stuck with casting the field to the type of the object the old fashioned way.
Simply casting the values to the right type should work:
(string) dr["someString"];
(int?) dr["someInteger"];
(byte[]) dr["somedata"];
string GetString(DataRow dr, string ColumnName)
{
if (dr.IsNull(ColumnName))
{
return null;
}
return (string)dr[ColumnName];
}
Another option is to use "as"
string str = dr["someString"] as string;
if it's DBNull.Value (or any other object not of type string), then str will get a real "null". Otherwise it will get the proper string value.
For value types, you can use nullable, i.e.
int? i = dr["someint"] as int?;
Again, it will get a real "null" instead of DBNull.Value. However, with nullable types you have to remember to use .Value, i.e.
int x = i.Value + 5;
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?
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;
}
}
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()
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.
You can use C#'s null coalescing operator
return accountNumber ?? string.Empty;
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);
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.
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 :)
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
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
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 :)
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);
}
I use an extension to eliminate this problem for me, which may or may not be what you are after.
It goes like this:
public static class Extensions
{
public String TrimString(this object item)
{
return String.Format("{0}", item).Trim();
}
}
Note:
This extension does not return null values! If the item is null or DBNull.Value, it will return an empty String.
Usage:
public string GetCustomerNumber(Guid id)
{
var obj =
DBSqlHelperFactory.ExecuteScalar(
connectionStringSplendidmyApp,
CommandType.StoredProcedure,
"GetCustomerNumber",
new SqlParameter("#id", id)
);
return obj.TrimString();
}
Convert it Like
string s = System.DBNull.value.ToString();