DapperLib/Dapper · error · ArgumentNullException

key

Error message

key

What it means

DapperRow.SetValue throws ArgumentNullException(nameof(key)) when the key is null. DapperRow is a dictionary-like row keyed by column name (resolved via the row's DapperTable), and a null key cannot be indexed. The same method also throws ArgumentException ("An item with the same key has already been added") on duplicate add, but the null-key guard fires first for null inputs.

Source

Thrown at Dapper/SqlMapper.DapperRow.cs:173

                if (index < 0 || index >= values.Length || values[index] is DeadValue) return false;
                values[index] = DeadValue.Default;
                return true;
            }

            object? IDictionary<string, object?>.this[string key]
            {
                get { TryGetValue(key, out object? val); return val; }
                set { SetValue(key, value, false); }
            }

            public object? SetValue(string key, object? value)
            {
                return SetValue(key, value, false);
            }

            private object? SetValue(string key, object? value, bool isAdd)
            {
                if (key is null) throw new ArgumentNullException(nameof(key));
                int index = table.IndexOfName(key);
                if (index < 0)
                {
                    index = table.AddField(key);
                }
                else if (isAdd && index < values.Length && !(values[index] is DeadValue))
                {
                    // then semantically, this value already exists
                    throw new ArgumentException("An item with the same key has already been added", nameof(key));
                }
                return SetValue(index, value);
            }
            internal object? SetValue(int index, object? value)
            {
                int oldLength = values.Length;
                if (oldLength <= index)
                {
                    // we'll assume they're doing lots of things, and

View on GitHub (pinned to 72a54c475f)

Solutions

1. Null-check the key before calling SetValue / before adding to the parameter dictionary.
2. Filter out null keys when building dictionaries from dynamic/untrusted sources.

Example fix

// before
row.SetValue(colName, val); // colName may be null

// after
if (colName is not null) row.SetValue(colName, val);

if (key is null) throw new ArgumentNullException(nameof(key));
row.SetValue(key, value);
// or when building a dictionary: filter null keys first

static bool IsValidKey(string? key) => key is not null;

try { row.SetValue(key, value); }
catch (ArgumentNullException ex) when (ex.ParamName == "key")
{ /* skip or default the null key */ }

When it happens

Trigger: Calling row.SetValue(null, value) directly; a dynamic insert/update path that produced a null column name; passing a dictionary with a null key into a Dapper execute that materializes a DapperRow parameter.

Common situations: Building parameter dictionaries dynamically where a key ended up null (e.g. a column name lookup returned null); deserializing JSON into a dictionary that allowed a null key then passing it to Dapper.

Related errors

• connection
• type
• type
• columnName
• type

AI-assisted analysis of DapperLib/Dapper@72a54c475f (2026-08-13). Data as JSON: /api/errors/5e756ec046260a8f. Report an issue: GitHub.