DapperLib/Dapper · error · InvalidOperationException

If specifying IsFixedLength, a Length must also be specifie

Error message

If specifying IsFixedLength,  a Length must also be specified

What it means

DbString.AddParameter throws InvalidOperationException when IsFixedLength is true but Length is still its default of -1. A fixed-length string (CHAR/NCHAR) requires an explicit size so the provider can size the parameter; without it the command would be malformed. The message (with its double space) is a known string in the Dapper source.

Source

Thrown at Dapper/DbString.cs:75

        public string? Value { get; set; }

        /// <summary>
        /// Gets a string representation of this DbString.
        /// </summary>
        public override string ToString() => Value is null
            ? $"Dapper.DbString (Value: null, Length: {Length}, IsAnsi: {IsAnsi}, IsFixedLength: {IsFixedLength})"
            : $"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 is not null && Value.Length <= DefaultLength)
            {
                param.Size = DefaultLength;

View on GitHub (pinned to 72a54c475f)

Solutions

1. Set an explicit Length matching the column size whenever IsFixedLength=true (e.g. Length = 50).
2. If the column is variable length, leave IsFixedLength=false (default) so Length=-1 is allowed.
3. Centralize DbString construction in a helper that asserts IsFixedLength implies a positive Length.

Example fix

// before
var p = new DbString { Value = "abc", IsFixedLength = true };
cnn.Execute("insert into T(C) values (@c)", new { c = p });

// after
var p = new DbString { Value = "abc", IsFixedLength = true, Length = 50 };

var p = new DbString { Value = s, IsFixedLength = true };
if (p.IsFixedLength && p.Length <= 0) throw new InvalidOperationException("Fixed-length DbString requires a positive Length");
cnn.Execute(sql, new { c = p });

try { cnn.Execute(sql, new { c = p }); }
catch (InvalidOperationException ex) when (ex.Message.Contains("IsFixedLength"))
{ /* set Length and retry / log config error */ }

When it happens

Trigger: Building a DbString, setting IsFixedLength=true, and never assigning Length (it defaults to -1); then passing that DbString as a parameter so AddParameter runs during command setup.

Common situations: Migrating dynamic SQL that used variable-length VARCHAR to fixed-length CHAR without updating parameter setup; copy-pasting a DbString config block and dropping the Length line; reading Length from config where the key was absent.

Related errors

• Member '{name}' is an {nameof(ICustomQueryParameter)} and ca

AI-assisted analysis of DapperLib/Dapper@72a54c475f (2026-08-13). Data as JSON: /api/errors/11df8af384f8e34c. Report an issue: GitHub.