Dapper and varchars

I found the following comment on the Dapper .NET project home page.

Dapper supports varchar params, if you are executing a where clause on a varchar column using a param be sure to pass it in this way:

    Query<Thing>("select * from Thing where Name = @Name", new {Name = 
    new DbString { Value = "abcde", IsFixedLength = true, Length = 10, IsAnsi = true });

On Sql Server it is crucial to use the unicode when querying unicode and ansi when querying non unicode

I'm evaluating Dapper for use with a legacy database (SQL Server 2008), with lots of stored procedures with varchar parameters, and I'm a little confused by this restriction.

With hand-crafted ADO.NET code, I'd use the following for the above query:

new SqlParameter("@Name", "abcde")

without specifying whether it's unicode or not, nor the length.

Why do I need this verbose DbString syntax with Dapper, specifying the column length, IsFixedLength and IsAnsi?
Why IsFixedLength = true for a varchar column (I'd expect it to be true for a char or nchar column)?
Do I have to use DbString like this for stored procedure parameters?

I was expecting Dapper to make my DAL code more concise, but this seems to be making it more verbose for varchar parameters.

UPDATE

I've researched a bit further, to try to understand why Dapper would have this varchar restriction, which I don't appear to have in my hand-crafted code, where I would normally create an input parameter as follows:

var parameter = factory.CreateParameter(); // Factory is a DbProviderFactory
parameter.Name = ...;
parameter.Value = ...;

and usually leave the provider to infer the DbType using its own rules, unless I specifically want to coerce it.

Looking at Dapper's DynamicParameters class, it has a method AddParameters which creates parameters as follows:

var dbType = param.DbType; // Get dbType and value
var val = param.Value;     // from 

...
// Coerce dbType to a non-null value if val is not null !!!!!
if (dbType == null && val != null) dbType = SqlMapper.LookupDbType(val.GetType(),name);
...
var p = command.CreateParameter();
...
if (dbType != null)                     
{                         
    p.DbType = dbType.Value;                     
}

I.e. it explicitly coerces IDataParameter.DbType to a value it looks up with its own algorithm, rather than leaving the provider to use its own rules.

Is there a good reason for this? It seems wrong for me, particularly in the light of the comment about Dapper's support for varchar parameters.

回答1:

You need this syntax when working with ODBC.

You would need to define a CHAR(30) field as a DbString in c# for Dapper and also set the length (30) and ansi (true) values to prevent Dapper from assuming the string was a text/blob type. Otherwise you will likely receive the error: "Illegal attempt to convert Text/Byte blob type".

I was getting this error using ODBC to connect to Informix until I defined my param as a DbString() and set the length and ansi values.

More info here.

回答2:

var param = new { Varchar1 = "", Varchar2 = "" };
db.Query("SP", param, commandType:CommandType.StoredProcedure);