DocumentTransactWrite.AddDocumentToUpdateHelper throws NullReferenceException for null ExpressionAttributeNames/Values

[steven-kehrli-netdocuments]

Describe the bug

The DocumentTransactWrite.AddDocumentToUpdateHelper method throws a NullReferenceException when either updateExpression.ExpressionAttributeNames, updateExpression.ExpressionAttributeValues, conditionalExpression.ExpressionAttributeNames, or conditionalExpression.ExpressionAttributeValues are null.

The TryFilterDuplicates method (called at lines 446 and 457 in DocumentTransactWrite.cs) does not perform null checking on its dictionary parameters before iterating over them, causing a NullReferenceException when these properties are null.

According to the DynamoDB Document Model API documentation and the Expression class design, these properties can be null (they are not marked as required/non-null), yet the SDK does not handle this case defensively.

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

The SDK should handle null ExpressionAttributeNames and ExpressionAttributeValues properties gracefully. When these properties are null, the SDK should treat them as "no attributes of this type" and continue processing without throwing exceptions.

The TryFilterDuplicates method should implement null-safe logic to handle cases where either or both dictionary parameters are null.

Current Behavior

A NullReferenceException is thrown when calling AddDocumentToUpdateHelper with an updateExpression or conditionalExpression that has null ExpressionAttributeNames or ExpressionAttributeValues properties.

Stack trace occurs in TryFilterDuplicates when attempting to iterate over a null dictionary.

Reproduction Steps

var updateExpression = new Expression
{
    ExpressionStatement = "SET #status = :status",
    ExpressionAttributeNames = new Dictionary<string, string> { ["#status"] = "Status" },
    ExpressionAttributeValues = null  // This causes the issue
};

var conditionalExpression = new Expression
{
    ExpressionStatement = "attribute_exists(Id)",
    ExpressionAttributeNames = new Dictionary<string, string> { ["#Id"] = "Id" },
    ExpressionAttributeValues = null  // Or this
};

var config = new TransactWriteItemsOperationConfig
{
    ConditionalExpression = conditionalExpression
};

// This throws NullReferenceException
transactWrite.AddDocumentToUpdate(..., updateExpression, config);

Possible Solution

private static bool TryFilterDuplicates<T>(
    Dictionary<string, T> src, 
    Dictionary<string, T> other,
    out Dictionary<string, T> dest, 
    out List<string> conflictingKeys)
{
    conflictingKeys = new List<string>();

    // If src is null, there's nothing to filter - return null
    if (src == null)
    {
        dest = null;
        return true;
    }

    // If other is null, all of src is unique - return src as-is
    if (other == null)
    {
        dest = src;
        return true;
    }

    // Both exist - filter for unique and conflicting keys as normal
    dest = new Dictionary<string, T>();
    // ... existing foreach logic
}

Additional Information/Context

No response

AWS .NET SDK and/or Package version used

AWSSDK.DynamoDBv2 4.0.8

Targeted .NET Platform

.NET 8.0

Operating System and version

macOS 15.x (Darwin 24.6.0)

[peterrsongg]

@steven-kehrli-netdocuments Thanks for opening up an issue.
I tried with the aws cli something like

aws dynamodb update-item --table-name TestTable --key '{\"Id\":{\"N\":\"1\"}}' --update-expression "SET Price = Price - :discount" --condition-expression "Price > :limit"

omitting the expression attribute values and it throws an exception

An error occurred (ValidationException) when calling the UpdateItem operation: Invalid UpdateExpression: An expression attribute value used in expression is not defined; attribute value: :discount

The docs don't explicitly say the expression attribute values are required, but this is the only way to pass in values to the update/condition expressions, so I'm not sure why you would set those values to null.

Are you just wanting to see a better exception message?

[peterrsongg]

@steven-kehrli-netdocuments Okay never mind, I was able to get it to work when trying a different update expression (like the one you were trying)

aws dynamodb update-item --table-name TestTable --key '{\"ID\": {\"N\": \"1\"}}' --update-expression "SET #price = Price" --expression-attribute-names '{\"#price\": \"Price\"}' --region us-west-2

this doesn't error out in the CLI. looks like a valid bug report

[steven-kehrli-netdocuments]

This is in the aws-sdk-net using the DynamoDB Document Model API. I can construct Expressions where I do not need to specify AttributeNames or AttributeValues and those operations work. This is in the DocumentTransactWrite TryFilterDuplicates code, where the sdk tries to dereference a potentially null dictionary.

[peterrsongg]

@steven-kehrli-netdocuments yeah i just wanted to see how the cli would work in your scenario to confirm it is a bug in our SDK.