DbString.cs

using System;
using System.Data;

namespace Dapper
{
    /// <summary>
    /// This class represents a SQL string, it can be used if you need to denote your parameter is a Char vs VarChar vs nVarChar vs nChar
    /// </summary>
    public sealed class DbString : SqlMapper.ICustomQueryParameter
    {
        /// <summary>
        /// Default value for IsAnsi.
        /// </summary>
        public static bool IsAnsiDefault { get; set; }

        /// <summary>
        /// A value to set the default value of strings
        /// going through Dapper. Default is 4000, any value larger than this
        /// field will not have the default value applied.
        /// </summary>
        public const int DefaultLength = 4000;

        /// <summary>
        /// Create a new DbString
        /// </summary>
        public DbString()
        {
            Length = -1;
            IsAnsi = IsAnsiDefault;
        }
        /// <summary>
        /// Ansi vs Unicode 
        /// </summary>
        public bool IsAnsi { get; set; }
        /// <summary>
        /// Fixed length 
        /// </summary>
        public bool IsFixedLength { get; set; }
        /// <summary>
        /// Length of the string -1 for max
        /// </summary>
        public int Length { get; set; }
        /// <summary>
        /// The value of the string
        /// </summary>
        public string Value { get; set; }
        
        /// <summary>
        /// Gets a string representation of this DbString.
        /// </summary>
        public override string ToString() =>
            $"Dapper.DbString (Value: '{Value}', Length: {Length}, IsAnsi: {IsAnsi}, IsFixedLength: {IsFixedLength})";

        /// <summary>
        /// Add the parameter to the command... internal use only
        /// </summary>
        /// <param name="command"></param>
        /// <param name="name"></param>
        public void AddParameter(IDbCommand command, string name)
        {
            if (IsFixedLength && Length == -1)
            {
                throw new InvalidOperationException("If specifying IsFixedLength,  a Length must also be specified");
            }
            bool add = !command.Parameters.Contains(name);
            IDbDataParameter param;
            if (add)
            {
                param = command.CreateParameter();
                param.ParameterName = name;
            }
            else
            {
                param = (IDbDataParameter)command.Parameters[name];
            }
#pragma warning disable 0618
            param.Value = SqlMapper.SanitizeParameterValue(Value);
#pragma warning restore 0618
            if (Length == -1 && Value != null && Value.Length <= DefaultLength)
            {
                param.Size = DefaultLength;
            }
            else
            {
                param.Size = Length;
            }
            param.DbType = IsAnsi ? (IsFixedLength ? DbType.AnsiStringFixedLength : DbType.AnsiString) : (IsFixedLength ? DbType.StringFixedLength : DbType.String);
            if (add)
            {
                command.Parameters.Add(param);
            }
        }
    }
}