Parameter size overwritten after custom TypeHandler

[mroth-omega]

When providing a custom TypeHandler for the type string, the IDbDataParameter.Size property is overwritten after calling the custom TypeHandler.SetValue method. The database provider I'm using does not accept nvarchar parameters with such a large size. The size property is being overridden by the block of code starting here:

Dapper/Dapper/SqlMapper.cs

Line 2844 in 6434c69

if (prop.PropertyType == typeof(string))

I am unable to create a pull request, but my suggested fix is to move that conditional block prior to calling the custom handler just above. Or, if necessary to set the size after setting the value, add a check for a valid size before overriding it with the DbString.DefaultSize constant loaded earlier in the function.

Here is an example type handler and setup code to reproduce the issue:

// Custom string type handler
private class StringTypeHandler : SqlMapper.TypeHandler<string>
{
    public override void SetValue(IDbDataParameter parameter, string? value)
    {
        int size = Math.Max(1, value?.Length ?? 100);
        parameter.DbType = DbType.String;
        parameter.Value = value;
        parameter.Size = size;
    }

    public override string? Parse(object value)
    {
        return value as string;
    }
}

// Replacing the default string type handler
SqlMapper.RemoveTypeMap(typeof(string));
SqlMapper.AddTypeHandler(new StringTypeHandler());

IDbConnection conn;
conn.Execute(cmd, new { p = "hello" })

I ran into this issue using Dapper version 2.1.35. My project is .NET 8 targeting win-x86 with an OdbcConnection using driver Microsoft Access Driver (*.mdb). Trying to execute a statement resulted in this error: System.Data.Odbc.OdbcException (0x80131937): ERROR [HY104] [Microsoft][ODBC Microsoft Access Driver]Invalid precision value. I can workaround this error by changing the type passed for parameters to use DbString. However, I would like to reuse a datatype that has string properties so I created a custom TypeHandler<string>. Through debugging, I found that the parameter size was set to 4000 when executing the command, despite my handler setting the size just moment earlier.